Chapter 13: Microbe-Human Interactions: Health and Disease

Vocabulary practice flashcards covering host-microbe interactions, normal resident microbiota, mechanisms of microbial virulence and pathogenicity, patterns and warning signs of disease, transmission dynamics, epidemiology, and Koch's postulates.

Infection

A condition in which pathogenic microorganisms penetrate host defenses, enter the tissues, and multiply.

Disease

Any deviation from health; a pathologic state that results from the cumulative effects of infection damage or disruption to tissues and organs.

Holobiont

A human host along with the trillions of resident microbes and transient organisms that colonize it, functioning together as an interactive unit.

Human Microbiome Project

A scientific project that revealed human cells contain approximately $21,000$ protein-encoding genes while the microbes inhabiting humans contain $8\text{ to }20\text{ million}$, populating many body sites previously thought to be sterile.

Virulence

The degree of pathogenicity of a microorganism, indicated by its ability to establish itself in a host and cause damage.

Virulence Factor

Any characteristic or structure of a microbe that contributes to toxin production or the induction of an injurious host response.

Infectious Dose (ID)

The minimum number of microbial cells or virus particles required for an infection to proceed; microbes with a smaller infectious dose have greater virulence.

Polymicrobial Infection

An infection involving contributions and damage from more than one microbial species simultaneously, such as influenza leading to bacterial pneumonia.

Portal of Entry

A characteristic route taken by a microbe to initiate infection in a host, usually through the skin or mucous membranes.

Exogenous Agent

An infectious agent originating from a source outside the host's body, such as the environment, another person, or an animal.

Endogenous Agent

An infectious agent already existing on or within the host's body, originating from normal resident biota or a previously silent infection.

Biosafety Levels (BSLs)

A system of four containment categories adopted by the Centers for Disease Control and Prevention (CDC) based on the degree of pathogenicity and relative danger of handling specific pathogens.

Biosafety Level 1 (BSL-1)

Facilities and practices used for low-infection-hazard microbes not generally considered pathogens in healthy adults (e.g., Micrococcus luteus, Bacillus megaterium, Lactobacillus, Saccharomyces), requiring standard open-bench protocols without special containment.

Biosafety Level 2 (BSL-2)

Facilities and practices required for Class 2 pathogens with moderate potential to infect healthy people (e.g., Staphylococcus aureus, Escherichia coli, Salmonella spp., hepatitis A and B viruses), requiring lab coats, gloves, posted biohazard signs, and biosafety cabinets when needed.

Biosafety Level 3 (BSL-3)

Facilities and containment practices for indigenous or exotic agents capable of causing severe or lethal disease especially via inhalation (e.g., Mycobacterium tuberculosis, Francisella tularensis, Yersinia pestis, yellow fever, HIV), requiring containment labs, biosafety cabinets for all manipulations, and protective clothing.

Biosafety Level 4 (BSL-4)

Maximum containment facilities designed for highly virulent microbes that pose extreme risk for aerosol-transmitted morbidity and mortality (e.g., Ebola virus, Marburg virus, Lassa fever virus), requiring complete isolation, clothing changes, and decontaminating chemical showers.

Phagocyte

A host white blood cell that engulfs and destroys pathogens by means of digestive enzymes and antimicrobial chemicals.

Antiphagocytic Factors

Virulence structures or capabilities utilized by pathogens to evade white blood cells, such as capsules that inhibit engulfment or mechanisms allowing survival inside the phagocyte.

Toxin

A specific chemical product of microbes, plants, or certain animals that is poisonous to other organisms.

Exotoxin

A toxic protein secreted by a living bacterial cell into infected tissues; it is toxic in minute amounts, specific to targeted host cells, heat-unstable at $60^\circ\text{C}$, and can be converted into a toxoid.

Endotoxin

Lipopolysaccharide (LPS) found exclusively in the outer membrane of gram-negative bacteria, released via shedding or lysis; it is toxic in high doses, heat-stable at $60^\circ\text{C}$, stimulates systemic fever, and cannot be converted into a toxoid.

Toxoid

An inactivated toxin used in vaccines that has lost its toxic effects but retains the ability to stimulate an immune response.

Antitoxin

An antibody that reacts specifically with and neutralizes a bacterial toxin.

Hemolysis on Blood Agar

The breakdown of red blood cells by bacterial hemolysins: beta (complete clearing zone), alpha (partial clearing/greenish discoloration), and gamma (no hemolysis).

Localized Infection

An infection pattern where a microbe enters the body and remains strictly confined to a specific tissue (e.g., boils, warts, fungal skin infections).

Systemic Infection

An infection that spreads to several sites and tissue fluids throughout the body, typically through the bloodstream, nerves, or cerebrospinal fluid.

Mixed Infection

An infection where several different microbial agents establish themselves simultaneously at the same site of infection.

Primary Infection

The initial infection that establishes itself in a host.

Secondary Infection

An infection that occurs when a primary infection is complicated or followed by an infection caused by a different microbe.

Acute Infection

An infection that comes on rapidly and has relatively short-lived effects.

Chronic Infection

An infection that progresses and persists over an extended period of time.

Sign

Any objective evidence of disease observed or measured by an examiner, providing more precise data than symptoms.

Symptom

Subjective evidence of disease as perceived and reported by the patient.

Syndrome

A disease identified or defined by a specific, characteristic complex of signs and symptoms.

Edema

The accumulation of excess fluid in afflicted tissues, representing a common sign of inflammation.

Granulomas and Abscesses

Walled-off collections of inflammatory immune cells and microbes within host tissues.

Lymphadenitis

The swelling and enlargement of one or more lymph nodes in response to infection.

Asymptomatic (Inapparent) Infection

An infection in which the host harbors multiplying microbes but experiences no perceptible symptoms and does not seek medical attention.

Portal of Exit

A specific avenue or route through which a pathogen vacates the host (e.g., respiratory droplets, saliva, skin scales, feces, urogenital discharges, blood).

Latency

A dormant state of an infectious agent during which it periodically reactivates to produce recurrent disease.

Sequelae

Long-term or permanent damage caused to tissues and organs as a consequence of an infection.

Reservoir

The primary habitat in the natural world from which a pathogen originates and continues to persist (such as human or animal carriers, soil, water, or plants).

Source

The distinct individual or object from which an infection is directly acquired by a host.

Asymptomatic Carrier

An individual who is infected with a pathogen and can transmit it to others, but exhibits no outward symptoms of disease (e.g., gonorrhea, genital herpes without lesions, HPV).

Incubating Carrier

An infected individual who spreads an infectious agent to others before displaying clinical symptoms (e.g., during the incubation period of infectious mononucleosis or COVID-19).

Convalescent Carrier

A recuperating patient who no longer exhibits symptoms but continues to shed viable microbes and transmit the infection to others (e.g., hepatitis A).

Chronic Carrier

An individual who shelters an infectious agent for an extended duration following recovery due to the latency of the microbe (e.g., tuberculosis, typhoid fever).

Passive Carrier

A healthcare or dental worker who mechanically transfers pathogens between patients via contaminated gloves, hands, or instruments without being infected.

Vector

In epidemiology, a live animal (predominantly an arthropod) that transmits an infectious agent from one host to another.

Biological Vector

A live vector that actively participates in a pathogen's life cycle, serving as a site for multiplication or development before communicating the agent to a human host.

Mechanical Vector

A vector that is not necessary to the pathogen's life cycle and merely transports the microbe externally without becoming infected itself.

Zoonosis

An infection indigenous to animals that is transmissible to humans, with humans typically acting as dead-end hosts who do not contribute to natural microbial persistence.

Communicable Disease

A disease that occurs when an infected host can transmit the infectious agent to another host and establish infection in that host.

Contagious

A descriptive property of an infectious agent that is highly communicable from person to person, especially through direct contact.

Noncommunicable Disease

An infectious disease that does not arise through transmission from host to host, often acquired from the environment or normal resident flora.

Vehicle

Any inanimate material commonly used by humans (such as air, water, soil, or food) that can transmit infectious agents.

Fomite

An inanimate object that harbors and transmits viable pathogens from one person to another but is not a continuous natural source of infection.

Oral-Fecal Route

A transmission route where an infected individual with poor personal hygiene contaminates food or water with feces, which is subsequently ingested by another host.

Droplet Nuclei

Dried microscopic residues created when microscopic pellets of mucus and saliva ejected from the mouth and nose evaporate and remain suspended in air.

Aerosols

Suspensions of fine dust or moisture particles in the air that contain viable pathogens.

Etiologic Agent

The specific causative microbial agent responsible for producing an infection or disease.

Koch's Postulates

A four-step experimental series of proofs developed by Robert Koch to determine the etiologic agent of an infectious disease by finding the microbe in all cases, isolating it in pure culture, reproducing the disease in a healthy subject, and reisolating the identical agent.

Molecular Koch's Postulates

A system of experimental criteria relying on molecular biology to establish that a specific gene within a pathogen contributes directly to its disease-causing capability.