Infection and Disease: Interactions Between Microbes and Humans

What is colonization?

Colonization is when microbes live in or on a host without causing disease.

What is an infection?

Infection occurs when microbes get past host defenses, enter tissues, and multiply, potentially causing inflammation.

What is disease?

Disease is when an infection damages or disrupts tissues or organs, causing a pathologic state.

What is commensalism?

Type of symbiosis where the microbe benefits but the host is unaffected.

What are examples of commensalism?

E. coli in the intestine or bactero on mucous membranes

What is mutualism?

Mutualism is a symbiotic relationship where both the host and microbe benefit.

What is an example of mutualism?

Protozoans in termites, bacteria in ruminant stomachs, lichens.

What is parasitism?

Parasitism is when a microbe benefits at the expense of the host, causing disease.

Obligate parasite

Must live in a host (viruses)

Facultative paraiste

Can live freely or in a host

Endoparasite

Lives inside a host

Ectoparasite

Lives on the outside of a host

Where are normal biota found in humans?

Found in the skin, mucous membranes, respiratory tract, gastrointestinal tract, urogenital tract.

Where are normal biota NOT found in humans?

Normally sterile sites include blood, cerebrospinal fluid, deep tissues, and organs like the heart, liver, and kidneys.

List factors that weaken host defenses and increase susceptibility to infection.

• Extremes of age (infancy, old age)

• Genetic or acquired immune defects (e.g., AIDS)

• Surgery or organ transplants

• Underlying disease (cancer, liver disease, diabetes)

• Chemotherapy or immunosuppressive drugs

• Physical or mental stress

• Pregnancy

• Other infections

Name 5 sources where newborns acquire microbiota.

• In utero (initial colonization)

• Birth canal (vaginal microbiota)

• Breastfeeding (milk microbiota)

• Contact with family members or caregivers

• Environment (skin, air, surfaces)

What is pathogenicity?

Pathogenicity is a microbe’s potential to cause disease.

What is virulence?

The degree to which a microbe can cause disease, determined by its ability to establish itself in a host and cause damage.

What is a virulence factor?

Any characteristic or structure that helps a microbe establish infection and cause damage.

What are polymicrobial infections?

Infections caused by multiple microbes.

What are examples are polymicrobial infections?

Influenza often leads to secondary pneumonia

What are the steps a microbe takes to cause disease?

1. Portal of entry

2. Adhesion

3. Surviving host defenses

4. Causing disease

5. Portal of exit

Portal of entry

Microbe enters the host

Adhesion

Microbe attaches to host cells

Surviving host defenses

Microbe evades immune system

Causing disease

Microbe damages tissues

Portal of exit

Microbe leaves host to infect others

What are common portals of entry?

Fecal-oral, inhalation, sexually transmitted, parenteral (through skin breaks).

What are portals of exit?

Shed through secretions, excretions, discharge, or sloughed tissue.

What is the infectious dose (ID)?

The minimum number of microbes needed to cause infection.

What is the affect of Smaller ID?

Higher virulence

EX: Q fever — 1 cell;

Cholera — 1 billion cells

How do bacteria, fungi, and protozoa attach to host cells?

• Fimbriae (pili)

• Surface proteins

• Adhesive slimes or capsules

How do viruses attach to host cells?

By specialized receptors.

How do parasitic worms attach to host cells?

Using suckers, hooks, or barbs (e.g., Taeniasis).

What are antiphagocytic factors?

Mechanisms microbes use to evade phagocytes:

Leukocidins

kill phagocytes

Slime or capsule:

Prevents engulfment

How do microbes cause tissue damage?

1. Enzymes or toxins

2. Inducing inappropriate/excessive host immune response

3. Epigenetic changes to host cells

What are exoenzymes?

Secreted enzymes that damage host tissues.

What are examples of exoenzymes?

• Muniase

• Hyalurondiase

• Coagulase

• Kinase

Muniase

Digests mucous membranes

Hyaluronidase

Digests connective tissue cement

Coagulase

Causes blood clotting

Kinase

Dissolves fibrin clots

What are exotoxins?

Small proteins secreted by bacteria that target specific host cells.

What are endotoxins?

Lipopolysaccharides (LPS) from Gram-negative bacteria outer membranes.

What is LIPS?

• Causes fever, inflammation, hemorrhage, diarrhea

• Can cause endotoxic shock (Salmonella, Shigella, Neisseria, E. coli)

What is a sign of disease?

Objective evidence observed by someone (e.g., fever, rash).

What is a symptom of disease?

Subjective evidence felt by the patient (e.g., pain, fatigue).

What is a syndrome?

A disease identified by a specific combination of signs and symptoms.

Signs of inflammation include:

• Edema (fluid accumulation)

• Granulomas or abscesses (walled-off infection)

• Lymphadenitis (swollen lymph nodes)

Signs of blood infection include:

• Leukocytosis: high WBC count

• Leukopenia: low WBC count

• Septicemia: microorganisms multiplying in blood

• Bacteremia/Viremia: microbes present in blood but not multiplying

What is an asymptomatic infection?

Infection without noticeable symptoms; host may not seek care.

What are the 5 stages of disease in a human?

1. Incubation period

2. Prodromal stage

3. Acute phase

4. Convalescent period

5. Continuation phase

Incubation period

Time from initial contact to first symptoms; microbe multiplies but no symptoms yet.

Prodromal stage

Early mild symptoms appear (e.g., fatigue, malaise, headache).

Acute phase

Infectious agent multiplies at high levels; prominent signs and symptoms.

Convalescent period

Host recovers; symptoms decline; strength gradually returns.

Continuation phase

Only some infections; organism lingers or symptoms continue after recovery.

What is a reservoir?

The primary habitat of a pathogen, where it normally lives and multiplies.

What is a transmitter?

An individual or object from which an infection is acquired.

What is zoonosis?

An infection naturally found in animals that can be transmitted to humans.

What are the 4 types of reservoirs?

1. Human reservoirs

2. Animal reservoirs

3. Nonliving reservoirs

4. Environmental reservoirs

Human reservoirs

Symptomatic or asymptomatic carriers (e.g., Thyroid carrier)

Animal reservoirs

Mammals (rabies), birds (psittacosis), arthropods (mosquitoes).

Nonliving reservoirs

Soil, water, plants (e.g., tetanus from soil)

Environmental reservoirs

Fomites, contaminated surfaces or objects

What is a communicable disease?

A disease in which an infected host can transmit the infectious agent to another host.

What is a contagious disease?

A highly communicable disease, easily spread via direct contact (e.g., influenza, measles).

What is a noncommunicable disease?

An infection that does not spread from host to host (e.g., tetanus, opportunistic infections).

What is horizontal transmission?

Disease spread from one individual to another in a population (direct, indirect, or vector).

What is vertical transmission?

Disease transmitted from parent to offspring via ovum, sperm, placenta, or milk.

What are carriers? Name 5 types.

Individuals who harbor and can transmit pathogens without showing disease.

1. Asymptomatic carrier

2. Incubatory carrier

3. Convalescent carrier

4. Chronic carrier

5. Passive carrier

Asymptomatic carrier

Infected but no symptoms

Incubatory carrier

Spreads disease during incubation period

Convalescent carrier

Spreads disease during recovery.

Chronic carrier

Harbors pathogen long-term

Passive carrier

Mechanically carriers pathogen without being infected

What is a healthcare-associated infection (HAI)?

Infection acquired during a hospital or healthcare stay.

What are two most common types of HAIs?

Pneumonia and urinary tract infections (UTIs).

What are common hospital pathogens?

• Clostridioides difficile: GI infections

• Staphylococcus aureus: pneumonia, surgical site infections, bloodstream infections

• Klebsiella spp.: UTIs, pneumonia, surgical site infections

• Escherichia coli: UTIs, bloodstream, surgical site infections

• Enterococcus spp.: UTIs, surgical site infections, bloodstream

What is epidemiology?

The study of the frequency and distribution of disease and health-related factors in populations.

How does epidemiology differ from traditional medical practice?

Epidemiology focuses on populations and disease trends, while traditional medicine treats individual patients.

What is a notifiable or reportable disease?

Diseases that must be reported to public health authorities (e.g., tuberculosis, COVID-19).

What is incidence?

Number of new cases in a defined time period ÷ number of susceptible persons

What is prevalence?

Total number of existing cases ÷ total population; a snapshot of disease at a given time.

What is the difference between incidence and prevalence?

Incidence measures new cases over time, prevalence measures all existing cases at a point in time

Define index case.

The first case that brings an epidemic to the attention of officials.

Define endemic.

A disease consistently present in a population at a steady rate.

Define sporadic.

Occasional cases occurring irregularly in random locations.

Define epidemic.

A sudden increase in the number of cases above what is normally expected.

Define pandemic.

An epidemic that spreads across continents or worldwide.

Define mortality rate.

The total number of deaths in a population due to a specific disease.

Define morbidity rate

The number of people afflicted with a disease in a population.