Chapter 21 - Respiratory System Infections (Nov. 20, 2025)

Bacterial disease

An illness caused by pathogenic bacteria that invade the body, multiply, and produce toxins or damage tissues, leading to symptoms such as fever, inflammation, pain, or impaired organ function. While many bacteria are harmless or beneficial, disease‑causing species can spread through contact, respiratory droplets, contaminated food or water, or insect vectors. These infections can affect various parts of the body, including the lungs, throat, skin, gut, or bloodstream, and range from mild conditions like strep throat to severe illnesses such as meningitis or sepsis. Treatment typically involves antibiotics, though resistance has become a major challenge, making prevention through hygiene, vaccination, and sanitation especially important.

Streptococcal pharyngitis (strep throat)

(Also known as strep throat) a bacterial infection of the throat and tonsils caused by Streptococcus pyogenes (Group A Streptococcus). It leads to symptoms such as sore throat, pain when swallowing, fever, red and swollen tonsils (sometimes with white patches), and swollen lymph nodes in the neck. The infection spreads through respiratory droplets and close contact, and unlike viral sore throats, it requires antibiotic treatment to prevent complications such as rheumatic fever or kidney inflammation. Diagnosis is usually confirmed with a rapid strep test or throat culture.

Streptococcus pyogenes

A Gram-positive, spherical bacterium that typically forms chains and belongs to Group A Streptococci (GAS). It is beta-hemolytic on blood agar, meaning it completely lyses red blood cells, and is catalase-negative and non-motile. This pathogen is responsible for a wide range of human diseases, from mild infections like strep throat and impetigo to severe conditions such as necrotizing fasciitis and streptococcal toxic shock syndrome. Transmission occurs through respiratory droplets or direct contact with infected wounds, and untreated infections can lead to serious complications like rheumatic fever or glomerulonephritis. Early diagnosis and antibiotic treatment are essential to prevent long-term sequelae.

Group A Streptococci (GAS)

A category of bacteria belonging to the species Streptococcus pyogenes. They are Gram‑positive, spherical bacteria that typically grow in chains and are known for causing a wide range of human diseases. Common infections include strep throat (streptococcal pharyngitis), scarlet fever, impetigo, and cellulitis, while more severe conditions can include rheumatic fever, necrotizing fasciitis, and toxic shock syndrome. GAS spread mainly through respiratory droplets or direct contact and are notable for their virulence factors, such as M protein and exotoxins, which help them evade the immune system and cause disease.

Virulence

The degree of pathogenicity of a microorganism, meaning how capable it is of causing disease in a host. It reflects the severity of illness the organism can produce and depends on factors such as toxins, enzymes, adhesion molecules, and immune‑evasion strategies. A highly virulent pathogen can cause serious or rapid disease, while a less virulent one may only produce mild symptoms or require special conditions to cause illness. In short, virulence describes the intensity of harm a microbe can inflict once it infects a host.

Virulence factor

A specific molecule, structure, or mechanism that a microorganism uses to cause disease in a host. These factors help pathogens invade tissues, damage cells, or evade the immune system, making them more capable of producing illness. Examples include toxins that harm host cells, capsules that prevent phagocytosis, adhesins that allow attachment to host surfaces, and enzymes that break down tissues. In short, a virulence factor is one of the tools a pathogen relies on to establish infection and increase its pathogenicity.

Streptolysin

A toxin produced by Streptococcus pyogenes (Group A Streptococcus) that damages host cells by creating pores in their membranes. This pore‑forming activity leads to cell lysis, particularly of red and white blood cells, contributing to tissue damage and weakening the immune response. There are two main types: Streptolysin O, which is oxygen‑sensitive and strongly immunogenic (used in diagnostic tests like ASO titers), and streptolysin S, which is oxygen‑stable and responsible for the beta‑hemolysis seen on blood agar plates. Together, streptolysins are important virulence factors that enhance the bacterium’s ability to cause disease.

Streptokinase

An enzyme produced by Streptococcus bacteria that acts as a virulence factor by helping the organism spread through host tissues. It works by activating plasminogen, a natural protein in the blood, and converting it into plasmin, which breaks down fibrin clots. By dissolving these clots, streptokinase allows the bacteria to move more freely through tissues and evade containment by the host’s defenses. Interestingly, purified streptokinase has also been used medically as a thrombolytic drug to dissolve dangerous blood clots in conditions like heart attacks, strokes, or pulmonary embolism, though newer agents are now more commonly used.

Bacterial capsule

A protective outer layer found on some bacteria, composed mainly of polysaccharides or polypeptides. It surrounds the cell wall and serves multiple functions: it helps bacteria evade the host immune system by inhibiting phagocytosis, aids in adherence to surfaces, and can protect against desiccation and antibiotics. Bacterial capsules are key virulence factors in pathogens like Streptococcus pneumoniae and Haemophilus influenzae, and their presence can be detected using special staining techniques in microbiology.

M Protein

A major virulence factor produced by Streptococcus pyogenes (Group A Streptococcus). It is a surface protein that helps the bacteria evade the host’s immune system by preventing phagocytosis and interfering with complement activation. M protein also aids in bacterial adhesion to host tissues, making infections like strep throat and skin infections more persistent. Because it is highly variable among strains, M protein contributes to the diversity of S. pyogenes and complicates vaccine development. Its presence is a key reason why Group A Streptococcus can cause both mild infections and severe diseases such as rheumatic fever.

Penicillin

A type of antibiotic that was first discovered by Alexander Fleming in 1928, derived from the Penicillium mold. It works by disrupting the formation of bacterial cell walls, which causes the bacteria to weaken and eventually die. This makes it especially effective against many common bacterial infections, such as strep throat, pneumonia, and skin infections. Penicillin revolutionized medicine by making previously deadly infections treatable. Although some bacteria have developed resistance over time, it remains a vital tool in modern healthcare.

Scarlet fever

A bacterial illness caused by Group A Streptococcus (Streptococcus pyogenes), the same bacterium responsible for strep throat. It typically affects children and presents with a sore throat, high fever, a characteristic red rash that feels like sandpaper, and a strawberry-like appearance of the tongue. The rash usually begins on the chest and spreads outward. Scarlet fever is contagious and spreads through respiratory droplets. With prompt treatment using antibiotics, it usually resolves without complications, but if left untreated, it can lead to serious conditions like rheumatic fever or kidney inflammation.

Erythrogenic toxin

An exotoxin produced by certain strains of Streptococcus pyogenes that acts as a superantigen, overstimulating the immune system and causing widespread release of inflammatory mediators. This immune overreaction damages blood vessels and leads to the characteristic red rash of scarlet fever, along with fever and systemic illness. The toxin does not generate red blood cells, despite its name, but instead causes the reddening of the skin. It is considered a major virulence factor in streptococcal infections because of its ability to provoke intense immune responses and contribute to disease severity.

Scarlet red tongue

A classic sign of scarlet fever, caused by Streptococcus pyogenes, where the tongue initially appears with a whitish coating and swollen red papillae (“white strawberry tongue”), then progresses as the coating peels away to reveal a bright red, inflamed, bumpy surface (“red strawberry tongue”). This vivid coloration results from vascular inflammation and the effects of erythrogenic toxin, making it a key diagnostic feature of the illness.

Necrotizing fasciitis

A severe bacterial infection that rapidly destroys the fascia (the connective tissue surrounding muscles, nerves, and blood vessels) and the subcutaneous tissue beneath the skin. It is most often caused by Streptococcus pyogenes (group A streptococcus), though other bacteria can be involved. The disease is sometimes called a “flesh‑eating infection” because of its aggressive tissue destruction. Symptoms include sudden, intense pain, swelling, fever, and skin changes that progress quickly. Necrotizing fasciitis is considered a medical emergency, requiring prompt surgical removal of infected tissue and broad‑spectrum antibiotics to prevent life‑threatening complications.

Pyrogenic exotoxin A

A potent superantigen produced by certain strains of Streptococcus pyogenes (group A streptococcus). It bypasses normal immune recognition and directly stimulates massive activation of T‑cells, leading to an uncontrolled release of cytokines. This immune overreaction causes fever, rash, shock, and multi‑organ involvement, and it is a key factor in severe conditions such as streptococcal toxic shock syndrome and sometimes scarlet fever. Because of its ability to provoke systemic inflammation, pyrogenic exotoxin A is considered one of the most dangerous streptococcal virulence factors.

Pyrogenic exotoxin B

A virulence factor produced by Streptococcus pyogenes (Group A Streptococcus). It is a cysteine protease enzyme that contributes to disease by breaking down host proteins, disrupting immune defenses, and damaging tissues. Unlike the superantigenic streptococcal pyrogenic exotoxins A and C, SpeB does not act as a superantigen; instead, it directly degrades structural and immune molecules such as immunoglobulins, complement proteins, and extracellular matrix components. This activity helps the bacteria spread through tissues and is linked to severe infections like necrotizing fasciitis and streptococcal toxic shock syndrome. In short, pyrogenic exotoxin B is a destructive protease that enhances the invasiveness of S. pyogenes.

Exoenzyme

An enzyme that is secreted by a microorganism into its external environment to break down large molecules—such as proteins, starches, or lipids—into smaller components that can be absorbed and used for nutrition or growth. These enzymes act outside the cell, helping microbes digest substances that are too big to pass through the cell membrane. Common examples include amylase, lipase, and protease, which are used by bacteria and fungi to degrade complex materials in their surroundings.

Post streptococcal sequelae

Medical complications that occur after an infection with Streptococcus pyogenes (Group A Streptococcus), even after the initial illness has resolved. They are not caused by the bacteria directly but by the body’s immune response, which mistakenly attacks its own tissues due to cross‑reactivity between bacterial antigens and host structures. Classic examples include rheumatic fever, which can damage the heart, joints, and nervous system, and post‑streptococcal glomerulonephritis, which affects the kidneys. These sequelae highlight how an untreated or severe strep infection can trigger long‑term health problems through immune‑mediated mechanisms.

Rheumatic fever

An inflammatory disease that can develop after an untreated or inadequately treated Group A Streptococcus (Streptococcus pyogenes) infection, such as strep throat or scarlet fever. It occurs when the immune system mistakenly attacks the body’s own tissues—especially the heart, joints, skin, and brain—in response to the bacterial antigens. Symptoms may include fever, painful and swollen joints, skin rash, and involuntary movements (chorea). The most serious complication is rheumatic heart disease, which can cause permanent damage to heart valves. Rheumatic fever is most common in children and adolescents and is preventable with prompt antibiotic treatment of strep infections.

Diphtheria

A serious bacterial infection caused by Corynebacterium diphtheriae, which primarily affects the throat and upper respiratory tract. It produces a potent exotoxin—only if the bacterium is infected by a specific virus—that can block protein synthesis in human cells, leading to complications like heart damage, nerve paralysis, and kidney failure. A hallmark symptom is the formation of a thick gray membrane in the throat, which can obstruct breathing. Diphtheria spreads through respiratory droplets and close contact, but is preventable through routine vaccination (DTaP or Tdap). Early treatment with antitoxin and antibiotics is crucial to reduce the risk of severe outcomes.

Corynebacterium diphtheriae

A Gram-positive, non-spore-forming, club-shaped bacterium that causes diphtheria, a potentially fatal disease marked by sore throat, fever, and a thick gray pseudomembrane in the throat. It becomes pathogenic when infected by a bacteriophage carrying the diphtheria toxin gene, enabling it to produce a cytotoxin that inhibits host cell protein synthesis. Transmission occurs via respiratory droplets, and the toxin can spread systemically, damaging the heart, nerves, and kidneys. Vaccination with diphtheria toxoid is highly effective in preventing infection, making it a cornerstone of public health immunization programs.

Pseudomembrane

A false membrane‑like layer that forms on mucosal surfaces during certain infections or inflammatory conditions. It is composed of dead cells, fibrin, inflammatory exudate, and sometimes bacteria, giving it a thick, grayish or yellowish appearance. Unlike a true tissue membrane, it is not a normal anatomical structure but rather a pathological deposit. Classic examples include the pseudomembrane seen in diphtheria (covering the throat and airway) and in Clostridioides difficile colitis (lining the colon). Its presence often signals severe tissue injury and can obstruct or impair normal function depending on the site.

Passive immunization

The process of providing immediate protection against a disease by transferring pre‑formed antibodies into a person, rather than stimulating their own immune system to produce them. These antibodies can come from human or animal sources (such as antiserum, immune globulin, or monoclonal antibodies) and offer short‑term defense because they are eventually broken down and do not create lasting immune memory. Passive immunization is often used in emergencies—such as after exposure to rabies, tetanus, or certain toxins—when rapid protection is needed before the body could mount its own immune response.

DTaP

A combination vaccine that protects against three serious bacterial diseases: diphtheria, tetanus, and pertussis (whooping cough). The name comes from the initials of each disease: D for diphtheria toxoid, T for tetanus toxoid, and aP for acellular pertussis components. It works by exposing the immune system to inactivated toxins (toxoids) and purified pertussis proteins, which safely stimulate immunity without causing illness. DTaP is typically given to children in a series of doses starting in infancy, providing strong protection against these potentially life‑threatening infections.

Pertussis

(Also commonly known as whooping cough) a highly contagious respiratory disease caused by the bacterium Bordetella pertussis. It is characterized by severe, prolonged coughing fits that can end with a high‑pitched “whoop” sound when the patient breathes in, especially in children. Other symptoms include runny nose, mild fever, and vomiting after coughing. Pertussis spreads through respiratory droplets and can be dangerous, particularly for infants, leading to complications such as pneumonia or seizures. Vaccination with DTaP (for children) or Tdap (for adolescents and adults) is the main preventive measure.

Bordetella pertussis

A small, Gram‑negative coccobacillus bacterium that causes pertussis (whooping cough), a highly contagious respiratory disease. It infects the lining of the respiratory tract and produces toxins, such as pertussis toxin, that damage cells and disrupt immune responses. The infection leads to severe, prolonged coughing fits, sometimes followed by a “whoop” sound when inhaling, especially in children. Transmission occurs through respiratory droplets, and while antibiotics can treat the infection, vaccination with DTaP or Tdap is the main preventive measure against this pathogen.

Ciliated cell

A type of epithelial cell that has tiny hair‑like projections called cilia on its surface. These cilia beat in coordinated waves to move substances across the cell’s surface, such as mucus, dust, or pathogens in the respiratory tract, or fluid in the reproductive system. In the airways, ciliated cells work with mucus‑secreting cells to trap and clear particles, forming part of the body’s first line of defense. Their rhythmic motion is essential for keeping passages clean and maintaining healthy tissue function.

Conjunctivitis

(Also known as pink eye) the inflammation of the conjunctiva, the thin, transparent membrane that lines the inside of the eyelids and covers the white part of the eye. It causes redness (the “pink” appearance), irritation, tearing, and sometimes discharge or swelling. Conjunctivitis can result from infections (viral or bacterial), allergic reactions, or irritants like smoke or chemicals. When infectious, it is often highly contagious, spreading through direct contact or contaminated surfaces. While usually mild and self‑limited, proper hygiene and treatment are important to prevent transmission and complications.

Otitis media

The inflammation or infection of the middle ear, the space behind the eardrum that contains tiny bones for hearing. It usually develops when bacteria or viruses spread from the nose or throat into the middle ear, often following a cold or respiratory infection. Fluid buildup behind the eardrum causes pressure, pain (earache), fever, and sometimes temporary hearing loss. While common in children due to their shorter, more horizontal eustachian tubes, it can occur at any age. Treatment may involve pain relief, antibiotics if bacterial, or watchful waiting, since many cases resolve on their own.

Sinusitis

The inflammation of the tissue lining the sinuses, which are air‑filled cavities around the nose and eyes. This condition is usually caused by infections (viral, bacterial, or rarely fungal), allergies, or structural blockages that prevent normal drainage. The inflammation leads to swelling, mucus buildup, and symptoms such as facial pain or pressure, nasal congestion, headache, loss of smell, and sometimes fever. Depending on how long it lasts, sinusitis is classified as acute (up to 4 weeks), subacute (4–12 weeks), or chronic (12 weeks or longer).

Antibiotic

A substance or drug that is used to kill or inhibit the growth of bacteria, helping to treat bacterial infections in humans, animals, or plants. Antibiotics work by targeting specific bacterial structures or functions, such as the cell wall, protein synthesis, or DNA replication. They are not effective against viruses, and misuse or overuse can lead to antibiotic resistance, where bacteria evolve to withstand the drug’s effects.

Bacterial pneumonia

A lung infection caused by pathogenic bacteria that invade the alveoli (air sacs) and surrounding tissue, leading to inflammation and fluid or pus accumulation. This disrupts normal gas exchange and produces symptoms such as fever, chills, cough with phlegm, chest pain, and difficulty breathing. Common bacterial culprits include Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus. The condition can range from mild to life‑threatening, especially in infants, the elderly, or those with weakened immune systems. Treatment typically involves antibiotics, supportive care, and prevention through vaccination and hygiene practices.

Pneumococcal pneumonia

A type of bacterial pneumonia caused by Streptococcus pneumoniae, a Gram‑positive bacterium with a protective capsule that helps it evade the immune system. It infects the alveoli (air sacs) in the lungs, leading to inflammation, fluid or pus accumulation, and impaired gas exchange. Symptoms typically include sudden fever, chills, chest pain, cough producing rust‑colored sputum, and difficulty breathing. This form of pneumonia is especially dangerous in young children, the elderly, and people with weakened immunity. Prevention relies on pneumococcal vaccines, while treatment involves antibiotics and supportive care.

Streptococcus pneumoniae

A Gram‑positive, encapsulated, spherical bacterium that often appears in pairs (diplococci) or short chains. It is a major human pathogen responsible for diseases such as pneumococcal pneumonia, meningitis, otitis media, and sinusitis. The capsule is its key virulence factor, helping it evade the immune system by resisting phagocytosis. Transmission occurs through respiratory droplets, and while it can colonize the nasopharynx harmlessly, under certain conditions it invades deeper tissues and causes serious infections. Prevention relies on pneumococcal vaccines, and treatment involves appropriate antibiotics.

Subunit vaccine

A type of vaccine that contains only specific, purified components of a pathogen—such as proteins, peptides, or polysaccharides—that are essential for triggering an immune response. Unlike whole-agent vaccines, subunit vaccines do not include the entire virus or bacterium, which makes them extremely safe, especially for immunocompromised individuals. Because they present only targeted antigens, they often require adjuvants to enhance the immune response and may need multiple doses or boosters to build strong immunity. Examples include the hepatitis B vaccine and HPV vaccine.

Capsule polysaccharide

A carbohydrate‑based protective layer that surrounds certain bacteria, forming part of their capsule. It is composed mainly of long chains of sugar molecules (polysaccharides) that create a slippery, gel‑like coating outside the cell wall. This structure helps bacteria evade the host’s immune system by resisting phagocytosis, preventing desiccation, and aiding in adherence to surfaces. Capsule polysaccharides are important virulence factors in pathogens such as Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae, and they are often used in vaccines to stimulate protective immunity.

Klebsiella pneumonia

A type of bacterial pneumonia caused by Klebsiella pneumoniae, a Gram‑negative, encapsulated rod‑shaped bacterium. It typically affects people with weakened immune systems, chronic illnesses, or those in hospital settings. The infection leads to severe inflammation and destruction of lung tissue, often producing thick, bloody, “currant jelly” sputum. Symptoms include high fever, chest pain, cough, and difficulty breathing. Because K. pneumoniae is often resistant to multiple antibiotics, treatment can be challenging and requires targeted antimicrobial therapy along with supportive care.

Klebsiella pneumoniae

A Gram‑negative, encapsulated, rod‑shaped bacterium belonging to the Enterobacteriaceae family. It normally lives in the human intestines without causing harm, but it can become an opportunistic pathogen in hospital or weakened‑immune settings. When invasive, it causes serious infections such as pneumonia, bloodstream infections, urinary tract infections, and wound infections. Its capsule polysaccharide is a major virulence factor, helping it resist phagocytosis and immune clearance. A major concern is that K. pneumoniae often shows multidrug resistance, making treatment difficult and requiring targeted antibiotics.

Carbapenem

A class of β‑lactam antibiotics known for their broad spectrum of activity and strong resistance to many bacterial enzymes (β‑lactamases) that normally inactivate other antibiotics. Structurally related to penicillins and cephalosporins, carbapenems work by binding to bacterial penicillin‑binding proteins, disrupting cell wall synthesis, and causing cell death. They are often reserved for severe or multidrug‑resistant infections, including those caused by Gram‑negative bacteria such as Klebsiella pneumoniae and Pseudomonas aeruginosa. Examples include imipenem, meropenem, ertapenem, and doripenem.

Carbapenemase

An enzyme produced by certain bacteria that breaks down carbapenem antibiotics, rendering them ineffective. Carbapenems are usually “last‑resort” drugs for multidrug‑resistant infections, so bacteria that produce carbapenemases (like Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii) pose a serious clinical threat. These enzymes belong to the broader group of β‑lactamases, but are especially powerful because they can hydrolyze carbapenems as well as other β‑lactam antibiotics. The presence of carbapenemase often leads to carbapenem‑resistant Enterobacteriaceae (CRE), which are difficult to treat and associated with high mortality.

Tuberculosis

A chronic infectious disease caused by Mycobacterium tuberculosis, a slow‑growing, acid‑fast bacterium. It most often affects the lungs (pulmonary TB) but can also spread to other organs (extrapulmonary TB). The disease develops when inhaled bacteria reach the alveoli, where they trigger an immune response that forms granulomas (tubercles) to contain the infection. Symptoms include persistent cough, chest pain, fever, night sweats, weight loss, and fatigue. TB can remain latent (dormant without symptoms) or progress to active disease, and while historically treated in sanatoriums, modern therapy relies on combinations of antibiotics such as isoniazid, rifampin, and streptomycin.

Mycobacterium tuberculosis

A slow-growing, rod-shaped, acid-fast bacterium that causes tuberculosis (TB), a contagious disease primarily affecting the lungs. It has a unique waxy cell wall rich in mycolic acids, which makes it resistant to many disinfectants and antibiotics, and requires special staining techniques for identification. The bacterium spreads through airborne droplets and can persist in a dormant state within host tissues for years, leading to latent infections. Symptoms of active TB include chronic cough, fever, night sweats, and weight loss, and treatment typically involves long-term antibiotic therapy due to its slow replication and potential drug resistance.

Tubercule

A small, rounded nodule or lump that can form in tissues. In medicine, the term most often refers to the granulomatous lesion produced during Mycobacterium tuberculosis infection. These tubercles are made up of immune cells—primarily macrophages, epithelioid cells, multinucleated giant cells, and lymphocytes—that cluster together to wall off the bacteria. Over time, the center may undergo caseous necrosis (cheese‑like tissue death), giving tubercles their distinctive appearance.

Tuberculin skin test

(Also known as the Mantoux test) a diagnostic procedure used to detect exposure to Mycobacterium tuberculosis, the bacterium that causes tuberculosis. In this test, a small amount of purified protein derivative (PPD), derived from the bacterium, is injected just under the skin of the forearm. After 48–72 hours, the site is examined for a raised, firm area of swelling (induration). The size of the induration, not the redness, determines whether the result is considered positive, which suggests prior infection or exposure. The TST does not distinguish between latent TB infection and active disease, so further tests are often required for confirmation.

Type IV hypersensitivity

A delayed immune response that occurs when sensitized T cells react to an antigen and trigger inflammation, usually 24 to 72 hours after exposure. Unlike other hypersensitivity types that involve antibodies, this reaction is mediated by T lymphocytes—especially Th1 cells—which release cytokines that recruit macrophages and other immune cells to the site. The resulting inflammation can cause tissue damage and is seen in conditions like contact dermatitis, the tuberculin skin test, and chronic transplant rejection. Because it relies on cellular activation rather than antibodies, the response is slower but can be more prolonged and destructive.

Multidrug resistant tuberculosis

A form of tuberculosis caused by Mycobacterium tuberculosis strains that are resistant to at least isoniazid and rifampin, the two most powerful first‑line anti‑TB drugs. This resistance makes treatment much more difficult, requiring longer courses of second‑line medications that are less effective, more toxic, and more expensive. MDR‑TB develops when TB treatment is incomplete, improperly prescribed, or not adhered to, allowing resistant bacteria to survive and multiply. It poses a serious global health challenge because it spreads like regular TB but is far harder to cure.

Legionnaire’s disease

A severe form of pneumonia caused by Legionella bacteria, most often Legionella pneumophila. The infection occurs when people inhale tiny water droplets (aerosols) containing the bacteria, which thrive in warm water systems such as cooling towers, hot tubs, and plumbing. Once in the lungs, the bacteria cause inflammation and fluid buildup, leading to symptoms like high fever, chills, cough, chest pain, muscle aches, and sometimes gastrointestinal issues. Legionnaire’s disease can be life‑threatening, especially in older adults, smokers, or those with weakened immune systems, but it is treatable with antibiotics if diagnosed promptly.

Legionella pneumophila

A Gram‑negative, rod‑shaped bacterium that is the primary cause of Legionnaire’s disease, a severe form of pneumonia. It thrives in warm, aquatic environments such as cooling towers, hot water tanks, air‑conditioning systems, and plumbing, where it can multiply inside free‑living amoebae. Infection occurs when people inhale contaminated water droplets (aerosols), allowing the bacteria to reach the lungs. L. pneumophila is an intracellular pathogen, meaning it can survive and replicate inside human macrophages, which makes it harder for the immune system to clear. It is also associated with a milder illness called Pontiac fever, which resembles flu‑like symptoms without pneumonia.

Viral disease

An illness caused by infection with a virus, which is a microscopic agent made of genetic material (DNA or RNA) enclosed in a protein coat. Because viruses cannot reproduce on their own, they must invade host cells and hijack their machinery to replicate. This process damages or disrupts normal cell function, leading to symptoms that vary depending on the virus and the tissues affected—ranging from mild colds and flu to serious conditions like HIV/AIDS, hepatitis, or COVID‑19. Viral diseases can spread through airborne droplets, direct contact, contaminated surfaces, insect vectors, or bodily fluids, and prevention often relies on vaccination, hygiene, and antiviral therapies.

Nasopharyngitis

(Also known as the common cold) an acute viral infection that causes inflammation of the nasal passages and the pharynx (the upper throat). It is most often triggered by rhinoviruses, though other respiratory viruses can also be responsible. Symptoms typically include nasal congestion, runny nose, sore throat, sneezing, mild cough, headache, and sometimes low‑grade fever. The illness is usually self‑limiting, resolving within a week to ten days, and treatment focuses on relieving symptoms rather than curing the infection.

Rhinovirus

A small, non‑enveloped RNA virus belonging to the Picornaviridae family, and it is the most common cause of the common cold. It primarily infects the upper respiratory tract, thriving at cooler temperatures found in the nasal passages. Transmission occurs through respiratory droplets, direct contact, or contaminated surfaces. Infection leads to symptoms such as sneezing, runny nose, sore throat, mild cough, and congestion, which usually resolve within a week. Although generally mild, rhinoviruses can trigger more severe illness in people with asthma or weakened immunity.

Coronavirus

A family of enveloped RNA viruses characterized by crown‑like spike proteins on their surface, responsible for a range of human and animal diseases. In people, coronaviruses can cause mild respiratory infections such as the common cold, but certain strains—like SARS‑CoV, MERS‑CoV, and SARS‑CoV‑2—lead to severe illnesses including SARS, MERS, and COVID‑19. They spread mainly through respiratory droplets, aerosols, or contact with contaminated surfaces, and their large RNA genome allows frequent mutations that can drive new outbreaks. Prevention relies on vaccination, hygiene, mask use, and ventilation, while treatment varies depending on the specific strain and severity.

Influenza

(Also known as the flu) a contagious viral respiratory illness caused by influenza viruses (types A, B, and C). It infects the nose, throat, and sometimes the lungs, leading to symptoms such as sudden fever, chills, cough, sore throat, muscle aches, fatigue, and headache. Unlike the common cold, influenza tends to be more severe and can cause serious complications like pneumonia, especially in young children, the elderly, and people with weakened immune systems. Transmission occurs mainly through respiratory droplets, and prevention relies on annual vaccination, hygiene practices, and antiviral medications when needed.

Swine flu

A respiratory disease caused by influenza viruses that normally infect pigs but can occasionally spread to humans. The most notable outbreak was in 2009, caused by the H1N1 influenza A virus, which contained a mix of genetic material from pig, bird, and human flu strains. In humans, swine flu produces symptoms similar to seasonal influenza—fever, cough, sore throat, body aches, fatigue, and sometimes gastrointestinal issues. Transmission occurs mainly through respiratory droplets, and while most cases are mild, severe illness can develop in vulnerable groups. Vaccination and antiviral medications are used to prevent and treat infection.

Bird flu

A viral infection caused by influenza A viruses that primarily affect birds but can occasionally infect humans and other animals. Wild waterfowl are natural reservoirs, and the virus can spread to domestic poultry, sometimes leading to large outbreaks. In humans, bird flu is rare but can be severe, with symptoms ranging from typical flu‑like illness (fever, cough, sore throat, muscle aches) to serious respiratory disease and even death. Transmission to people usually occurs through direct contact with infected birds or contaminated environments, rather than sustained person‑to‑person spread. Certain strains, such as H5N1 and H7N9, are of particular concern due to their high mortality rates and pandemic potential.

Antigenic drift

The gradual, continuous change in viral surface proteins (antigens) caused by small mutations in the virus’s genetic material during replication. These mutations accumulate over time, altering proteins like hemagglutinin and neuraminidase in influenza viruses, which helps the virus evade recognition by the host’s immune system. Because of antigenic drift, people can be reinfected with influenza multiple times, and vaccines must be updated regularly to match circulating strains. It is a key reason why influenza remains a persistent public health challenge.

Antigenic shift

The sudden, major change in a virus’s surface antigens caused by genetic reassortment, usually when two different strains of influenza A infect the same host and exchange segments of their RNA genome. This produces a new subtype with novel hemagglutinin and/or neuraminidase proteins that the human immune system has little to no prior recognition of. Because of this, antigenic shift can lead to widespread outbreaks or pandemics, as seen with the 1957 Asian flu and the 2009 H1N1 swine flu. Unlike antigenic drift, which is gradual and continuous, antigenic shift is abrupt and rare but far more dangerous in terms of public health impact.

Respiratory syncytial virus

A common, highly contagious RNA virus that infects the respiratory tract, especially in infants, young children, and older adults. It spreads through respiratory droplets and direct contact, causing symptoms similar to a cold—runny nose, cough, fever, and wheezing—but can progress to serious illnesses like bronchiolitis or pneumonia in vulnerable groups. RSV is notable for its ability to cause syncytium formation, where infected cells fuse together, disrupting normal lung tissue function. Most infections are mild and self‑limiting, but severe cases may require hospitalization, making RSV a major cause of seasonal respiratory illness worldwide.

Syncytium

A multinucleated cell‑like structure formed when individual cells fuse together, sharing a common cytoplasm and membrane. This process can occur naturally, such as in skeletal muscle fibers, or pathologically, as in viral infections where viruses like respiratory syncytial virus induce cell fusion. Syncytia disrupt normal tissue architecture and are often used as diagnostic markers in virology and pathology. They differ from coenocytes, which arise from nuclear division without cell fusion.

Croup

A respiratory illness in young children caused most often by viral infection (commonly parainfluenza viruses) that leads to swelling of the larynx, trachea, and bronchi. This inflammation produces the characteristic “barking” cough, hoarseness, and stridor (a harsh, high‑pitched sound when breathing in). Symptoms often worsen at night and can range from mild to severe, sometimes requiring medical attention if breathing becomes difficult. Croup spreads through respiratory droplets and usually resolves within a few days, though supportive care such as humidified air or corticosteroids may be used in more serious cases.

Hantavirus

A group of rodent‑borne RNA viruses that can cause severe human disease, most notably hantavirus pulmonary syndrome (HPS) in the Americas and hemorrhagic fever with renal syndrome (HFRS) in Europe and Asia. People usually become infected by inhaling dust contaminated with rodent urine, droppings, or saliva, and person‑to‑person transmission is extremely rare. Early symptoms resemble the flu—fever, muscle aches, and fatigue—but HPS can quickly progress to respiratory failure, while HFRS primarily affects the kidneys. Mortality rates vary widely, from about 1% in milder HFRS cases to up to 40% in HPS, making hantavirus a serious public health concern. Prevention focuses on rodent control and minimizing exposure to contaminated environments.

Hantavirus pulmonary syndrome

A severe respiratory illness caused by hantaviruses carried by rodents, most often the deer mouse in North America. Infection occurs when people inhale dust contaminated with rodent urine, droppings, or saliva, and the disease begins with flu‑like symptoms such as fever, muscle aches, and fatigue. Within days, it can progress to coughing, shortness of breath, and acute respiratory failure due to fluid buildup in the lungs. With a mortality rate of about 30–60%, HPS is considered a serious public health threat, and since no specific treatment or vaccine exists, prevention relies on rodent control and avoiding exposure to contaminated environments.

Fungal disease

An infection caused by pathogenic fungi that can affect humans, animals, or plants, ranging from mild superficial conditions like athlete’s foot and ringworm to serious systemic illnesses such as histoplasmosis or aspergillosis. These diseases occur when fungi invade tissues, often through inhalation, ingestion, or direct contact, and are more severe in individuals with weakened immune systems. Symptoms vary depending on the site of infection, from itching and scaling of the skin to fever, cough, and organ dysfunction in systemic cases. Treatment relies on antifungal medications, but resistance is an emerging concern, making prevention and awareness important for public health.

North American blastomycosis

A fungal infection caused by Blastomyces dermatitidis, typically acquired by inhaling spores from moist soil, decaying wood, or organic matter in endemic regions of the United States and Canada. It most often begins in the lungs, producing pneumonia‑like symptoms such as fever, cough, chest pain, and night sweats, but can spread to the skin, bones, and other organs, forming chronic lesions. While some infections remain mild, others can become severe or disseminated, especially in immunocompromised individuals. Treatment requires antifungal medications such as itraconazole for moderate cases or amphotericin B for more serious disease, making early recognition and management essential.

Blastomyces dermatitidis

Dimorphic fungus found in moist soil and decaying organic matter, especially near rivers and lakes in parts of North America. In the environment it grows as a mold producing infectious spores, but once inhaled into the lungs it converts into a thick‑walled yeast with broad‑based budding, a hallmark diagnostic feature. This organism causes blastomycosis, a disease that often begins with pneumonia‑like symptoms and can spread to the skin, bones, and other organs if untreated. Because it is not transmitted person‑to‑person, infection occurs only through environmental exposure, and treatment requires antifungal medications such as itraconazole or amphotericin B.

Skin ulcer

An open sore that develops when the skin and underlying tissue break down, often due to poor blood circulation, prolonged pressure, or infection. It typically appears as a painful, inflamed area that may ooze fluid, form a crust, or fail to heal properly. Common causes include conditions like diabetes, venous insufficiency, or immobility, and if untreated, ulcers can deepen and lead to serious complications such as infection or tissue death.

Tissue ulcer

An open sore that develops when the skin or deeper tissues break down due to poor blood supply, prolonged pressure, infection, or injury. It appears as a lesion that fails to heal, often exposing underlying layers, and can occur in places like the mouth, stomach, or on the skin. If untreated, tissue ulcers may deepen, spread, and lead to complications such as infection or necrosis.

Amphotericin B

A powerful antifungal medication used to treat serious systemic fungal infections, such as histoplasmosis, blastomycosis, and cryptococcosis. It works by binding to ergosterol in fungal cell membranes, creating pores that cause leakage of cell contents and ultimately cell death. Because it can also affect human cell membranes, amphotericin B is effective but often associated with significant side effects, including kidney toxicity and infusion reactions, so its use is carefully monitored.