Infectious Diseases: Key Concepts and Terminology

Introduction to Infectious Diseases

Basic Concepts of Infection

  • Host: Human or animal colonized by a pathogen.

  • Pathogen: Microorganisms capable of causing infectious disease.

  • Colonization: Pathogen living in a host.

  • Infection: Invasion, colonization, and multiplication of pathogens in a host.

  • Virulence: Ability of a pathogen to produce disease.

  • Reservoir: Source of a pathogen; examples include infected persons or inanimate objects (fomites).

  • Vector: Living beings that can carry pathogens from reservoir to host (e.g., mosquitoes, ticks, fleas).

Example of Infection Transmission Cycle

  • Reservoir hosts: Birds.

  • Insect vector: Culex mosquitoes transfer pathogens between birds and humans.

  • Accidental hosts: Humans and animals exposed to the pathogens.

Epidemiology Terms

  • Epidemiology: Study of disease distributions.

  • Incidence: Number of new infection cases occurring in a specific timeframe.

  • Prevalence: Number of active ongoing infections at a specific moment.

  • Endemic: A situation where incidence and prevalence rates are stable.

  • Epidemic: A sudden increase in the incidence of a disease in a given area.

  • Pandemic: Global spread of disease affecting multiple countries or continents.

Normal Microbial Flora

  • Microbial flora: Organisms living on or in the human body that perform beneficial functions:

    • Secretion of nutrients

    • Competitive inhibition of harmful pathogens

    • These organisms typically do not cause disease unless they enter sterile areas.

Immunity

  • Immunocompetence: The ability to protect oneself from infections.

  • Immunosuppression: A defect in the immune system that increases susceptibility to infections.

  • Opportunistic infections: Infections that occur when the immune system is weakened.

Hospital-acquired Infections

  • Nosocomial: Infections originating in clinical environments, common examples include:

    • Catheter-associated urinary tract infection (CAUTI)

    • Ventilator-associated pneumonia (VAP)

    • Methicillin-resistant Staphylococcus aureus (MRSA)

    • Clostridium difficile (C. diff)

    • Surgical site infections

    • Central-line-associated bloodstream infections

Types of Microorganisms

Bacteria

  • Characteristics:

    • Unicellular prokaryotes classified by shape, oxygen requirement (aerobic or anaerobic), and laboratory staining methods such as the Gram stain.

    • For example: Neisseria gonorrhoeae is classified as N. gonorrhoeae.

Gram Stain

  • The most common clinical microbiology stain:

    • Results based on bacterial cell wall composition:

    • Gram-positive: Thick peptidoglycan wall; appears purple after staining.

    • Gram-negative: Thin cell wall with outer membrane; appears pink after staining.

Mycobacterium

  • Examples include: Mycobacterium tuberculosis and Mycobacterium leprae.

  • These have waxy cell walls making them difficult to stain and kill.

Viruses

  • Characteristics:

    • Acellular and rely on host cells for replication.

    • Composed of genetic material (DNA or RNA) surrounded by a protein coat.

    • Can cause various conditions including acute infections, chronic infections, latent infections, and cancer.

Fungi

  • Characteristics:

    • Includes mold-like organisms, known to cause mycoses (fungal infections) that can be deep or superficial.

    • Examples:

    • Dermatophytes (like Tinea, ringworm)

    • Candida albicans, which can cause severe disseminated infections in immunocompromised patients.

Protists & Helminths

  • Protists: Unicellular eukaryotes such as protozoa, causing diseases like Giardiasis and malaria.

  • Helminths: Parasitic worms that can cause infections in humans.

Prions

  • Proteinaceous infectious agents that lead to neurodegenerative diseases in humans and animals, e.g., Bovine spongiform encephalopathy (mad cow disease).

Mechanism of Infection

Immune Mechanism

  • Pathogens can overwhelm host's defensive barriers.

  • The immune system has two primary levels:

    • Innate immunity: Nonspecific defenses (skin, stomach acid).

    • Adaptive immunity: Specificity through sensitized T and B lymphocytes.

Chain of Infection

  • Key components include:

    • Infectious agent

    • Susceptible host

    • Portal of entry

    • Mode of transmission

    • Reservoir

    • Portal of exit

Portal of Entry

  • Defenses:

    • Skin: Physical barrier, hosts normal flora.

    • Respiratory tract: Potential entry point for numerous microorganisms, protected by cilia and specialized immune cells.

    • Gastrointestinal tract: Protects through stomach pH and normal flora.

    • Genitourinary tract: Urethra can be a common route for infections.

    • Blood-blood transmission: Includes transfusions, needle sharing.

    • Maternal-fetal transmission: Certain pathogens can cross the placenta (e.g., CMV, rubella).

Stages of Infection

  • Incubation stage: Period when microorganisms replicate without symptoms, lasting 24 hours to 2-3 months.

  • Prodromal stage: Initial vague symptoms appear.

  • Acute stage: Full infectious disease with clear signs and symptoms.

  • Convalescent stage: Body begins to eliminate the pathogen.

  • Resolution stage: Pathogen eliminated from the body.

Diagnosis of Infection

  • Signs and Symptoms: E.g., jaundice in hepatitis.

  • Laboratory studies:

    • Staining (e.g., Gram stain)

    • Culturing organisms

    • Biopsy for histological examination

    • Serological tests for antibodies (antibody titer levels)

    • Direct antigen identification

    • Polymerase chain reaction (PCR) test

    • Complete blood count (CBC): High WBC can indicate infection.

Treatment and Eradication of Infection

  • Options:

    • Self-limiting infection, antimicrobial drugs, immune-boosting agents, surgical removal of infected tissues.

  • Prevention: Vaccination against certain infections.

Emerging Infectious Diseases

  • Characteristics:

    • Increasing incidence or geographic range.

    • Recognition of previously undetected infections.

    • Established diseases previously believed not to be infectious may now have origins identified.

Additional Infection Terms

  • Reemerging infections: Infections that have declined but are now at higher levels (e.g., malaria, tuberculosis).

  • Zoonotic infections: Diseases passed from animals to humans (e.g., HIV, Ebola).

  • Bioterrorism infectious agents: Infectious agents used as weapons (e.g., anthrax).

Knowledge Check

  • During which stage of infection is a person MOST contagious?

  • Options: Incubation, Prodromal, Acute, Convalescence.

Bacterial Infections: Staphylococcal

General Information

  • Staphylococcus species: Gram-positive, part of normal flora.

  • Opportunistic: S. aureus colonizes numerous skin and mucosal surfaces.

  • Most virulent: S. aureus infections attributed to its ability to produce toxins and proteases.

Antibiotic Resistance

  • MRSA: Methicillin-resistant Staphylococcus aureus.

  • VRSA: Vancomycin-resistant Staphylococcus aureus.

Symptoms of Staph Infection

  • Signs include:

    • Confusion, painful skin, lumps, altered breathing patterns, redness or warmth of skin, swelling, fever, rash, diarrhea, or vomiting.

Diagnosis and Complications

  • Diagnosis through pus-filled lumps, cultures, and nasal swabs for MRSA detection.

  • Complications: May lead to serious conditions such as sepsis and toxic shock syndrome (TSS).

Bacterial Infections: Streptococcal

General Information

  • Characteristics: Gram-positive, multiple infectious strains.

  • Notable strains: S. pyogenes causes strep throat and scarlet fever.

Virulence Factors

  • Secretes: Virulence factors including streptolysins and exotoxins.

  • Associated diseases: May lead to rheumatic fever and glomerulonephritis.

Signs & Symptoms of Strep Throat

  • Symptoms include: cough, headache, fever, weakness, swollen tonsils, and development of a strawberry tongue.

Diagnosis and Complications

  • Diagnosis through throat culture and CBC.

  • Complications include rheumatic fever and ear/sinus infections.

Rheumatic Fever

  • Causes: Arises from untreated strep throat.

  • Symptoms: Include joint pain, heart murmur, skin rash, and Sydenham's chorea (involuntary movements).

Bacterial Infections: Pneumonia

General Information

  • Primary Agent: Streptococcus pneumoniae is the most common cause of community-acquired pneumonia.

  • Gram-positive diplococci.

Antibiotic Resistance

  • Resistance is common; hence antibiotic susceptibility testing is critical.

Symptoms and Diagnosis

  • Symptoms include shortness of breath (SOB), chest pain, cough, and fever.

  • Diagnosis via imaging (x-ray) and CBC.

Types of Pneumonia

  • Mycoplasma pneumoniae: Lacks a cell wall, causing “walking pneumonia.”

  • Legionnaire’s Disease: Caused by Legionella pneumophilia; associated with contaminated water and environmental conditions.

Bacterial Infections: Pseudomonas

General Information

  • Pathogenicity: Pseudomonas aeruginosa is a Gram-negative, opportunistic pathogen found commonly in hospitalized patients.

  • Resistance: Highly resistant to many antibiotics.

Symptoms and Complications

  • May result in serious infections such as pneumonia, UTIs, and necrotizing skin infections.

Bacterial Meningitis Infection

General Information

  • Nature: Can be fatal within 24-48 hours and is more severe than viral meningitis.

  • Common symptoms include fever, neck stiffness, headache, and photophobia.

Diagnosis

  • Diagnosis often requires a lumbar puncture; includes assessment of Kernig’s and Brudzinski’s signs.

Common Pathogens

  • Neisseria meningitidis: Transmitted via droplets, may cause petechiae and gangrene.

  • Haemophilus influenzae: Gram-negative, subtype b is the most virulent.

Gastrointestinal Bacterial Infections

Key Pathogens

  • Salmonella: S. enterica can cause gastroenteritis; S. typhi can lead to typhoid fever.

  • Shigella: Can lead to severe bloody diarrhea and hemolytic uremic syndrome.

Control and Treatment

  • Escherichia coli (E. coli): Various strains exist, with E. coli O157:H7 causing outbreaks.

  • Campylobacter jejuni: Often self-limiting but may lead to Guillain-Barre syndrome.

Antimicrobial Resistance

Overview

  • Bacteria can develop resistance through genetic mutation and horizontal transfer, commonly due to overuse and misuse of antibiotics.

Implications

  • Increased healthcare costs, higher mortality rates, and prevalence of difficult-to-treat infections.

MRSA & C. difficile

MRSA

  • General Information: Resistant to beta-lactam antibiotics, difficult to treat.

  • Symptoms: Signs of skin infection include redness, swelling, and pus.

C. difficile

  • General Information: Mostly associated with antibiotic use leading to GI overgrowth.

  • Symptoms: Characterized by severe diarrhea, abdominal pain, and may lead to toxic megacolon.

Treatment Options

  • MRSA: Treated with vancomycin; caution for renal failure and fast IV infusion reactions.

  • C. difficile: Treated with specific antibiotics, including metronidazole and vancomycin.

Tuberculosis

Overview

  • A chronic lung infection that can remain latent or become active under immunosuppression.

  • Symptoms include chronic cough, bloody sputum, fever, and night sweats.

Treatment

  • Highly resistant strains require a multi-drug approach including Rifampin and Isoniazid for effective treatment.

Viral Infections

Common Viral Infections

  • Influenza: Characterized by abrupt symptoms and high mutation rate leading to annual vaccines; symptoms include cough, fever, and myalgia.

  • COVID-19: SARS-CoV-2 virus with symptoms such as fever, fatigue, and pneumonia risk.

Additional Viral Infections

  • Includes Epstein-Barr Virus (EBV), Cytomegalovirus (CMV), rubella, and Varicella Zoster (chickenpox).

Antiviral Medications

Classes of Antiviral Drugs

  • Acyclovir: For herpes simplex; toxicity includes nephrotoxicity and CNS effects.

  • Oseltamivir: Effective for influenza; should be administered within 48 hours of symptom onset.

HIV/AIDS

Overview

  • Human Immunodeficiency Virus (HIV) targets CD4 cells, leading to Acquired Immunodeficiency Syndrome (AIDS) defined by a CD4 count < 200 or opportunistic infections.

Transmission

  • Dependent on viral load and can be transmitted through blood, sexually, and mother-to-fetus.

Treatment

  • Combination antiretroviral therapy (ART) is recommended to minimize resistance.

Fungal Infections

Common Fungal Pathogens

  • Candida albicans: Overgrowth can lead to infections such as thrush and vulvovaginal yeast infections.

  • Aspergillus spp.: Can cause pulmonary disease, especially in the immunocompromised.

Antifungal Medications

Common Classes

  • Amphotericin B: Targets fungal cell membranes, with nephrotoxicity as a major side effect.

  • Azoles: Good oral absorption but extensive drug interactions requiring liver function monitoring.

Parasitic Infections

Common Parasites

  • Malaria: Caused by Plasmodium spp. and transmitted by Anopheles mosquitoes.

  • Toxoplasmosis: Transmitted via cat feces; may cause severe infections in immunocompromised individuals.

Treatment Options

  • Metronidazole for giardiasis; Chloroquine for malaria treatment and prevention.

Conclusion

Key Concepts

  • Infection mechanisms, types of pathogens, and antimicrobial treatments are crucial in understanding infectious diseases. Monitoring resistance, effective diagnosis, and prompt treatment are essential in managing infections effectively.