Infection, Infectious Disease, and Epidemiology (Chapter 14) – Microbiology Study Notes

Infection, Infectious Disease, and Epidemiology

Introduction

This chapter explores how microbes interact with hosts, the mechanisms by which they cause disease, and how infectious diseases are classified and spread. It also introduces the principles of epidemiology, which is the study of disease occurrence and transmission in populations.

Symbiotic Relationships Between Microorganisms and Humans

Types of Symbiosis

• Symbiosis means "to live together" and refers to close associations between different species.

• Humans have symbiotic relationships with countless microorganisms, with microbial cells outnumbering human cells in the body.

Type	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Neither benefits nor is harmed	Mites in human hair follicles
Amensalism	Is harmed	Neither benefits nor is harmed	Fungus secreting antibiotic, inhibiting nearby bacteria
Parasitism	Benefits	Is harmed	Tuberculosis bacteria in human lung

Examples of Symbiosis

• Mutualism: Both partners benefit. Example: Bacteria in the colon produce vitamins for the host and receive nutrients and a stable environment.

• Obligate Mutualism: Some relationships are so essential that neither organism can survive without the other (e.g., termites and wood-digesting protozoa).

• Commensalism: One organism benefits, the other is unaffected. Example: Demodex folliculorum mites live on human skin without causing harm.

• Amensalism: One organism is harmed, the other is unaffected. Example: Penicillium fungus produces penicillin, inhibiting bacteria but not being affected itself.

• Parasitism: The parasite benefits at the expense of the host. Pathogens are parasites that cause disease (e.g., protozoa, fungi, bacteria).

The Human Microbiome

Microbiome of Humans

• Microbes that colonize the body without causing disease are called the normal microbiota or normal flora.

• The microbiome includes resident microbiota (present for most of life) and transient microbiota (present temporarily).

Type	Description
Resident microbiota	Remain a part of the microbiome for most of a person's life.
Transient microbiota	Remain in the body for only a few hours, days, or months before disappearing.

• Acquisition of the normal microbiota begins at birth; the womb is typically axenic (free of microorganisms).

Opportunistic Pathogens

Normal microbiota can become harmful if certain conditions arise:

• Introduction into unusual sites (e.g., E. coli from colon to urinary tract)

• Immune suppression (e.g., HIV, immunosuppressive drugs)

• Changes in the normal microbiota (e.g., long-term antibiotics)

• Stressful conditions (e.g., metabolic or emotional stress)

Reservoirs of Infection

Types of Reservoirs

• Animal reservoirs: Diseases maintained in animals and transmitted to humans (zoonoses).

• Human carriers: Infected individuals who may be asymptomatic but can transmit disease (e.g., Typhoid Mary).

• Nonliving reservoirs: Soil, water, and food contaminated with pathogens.

Zoonoses are diseases that spread naturally from animal hosts to humans via direct contact, consumption, or arthropod vectors.

Exposure to Microbes: Contamination and Infection

Contamination vs. Infection

• Contamination: Presence of microbes on or in the body (e.g., eating, drinking, swimming, bug bites).

• Infection: When a microbe evades defenses, multiplies, and becomes established in the body.

Portals of Entry

• Skin

• Mucous membranes

• Placenta

• Parenteral route (injection or breakage of skin)

Infection and Disease

Definitions

• Infection: Invasion of the host by a pathogen.

• Disease: Condition resulting from infection or other factors that disrupt normal body function or structure.

• Symptoms: Subjective characteristics felt by the patient (e.g., pain, dizziness).

• Signs: Objective manifestations observed or measured (e.g., swelling, fever).

• Syndrome: Group of symptoms and signs that characterize a disease.

• Asymptomatic (subclinical) infections: Lack symptoms but may show signs.

Etiology and Koch's Postulates

• Etiology: Study of the cause of disease.

• Germ theory of disease: Diseases are caused by infections of pathogenic microorganisms.

• Koch's Postulates: Criteria to prove a specific pathogen causes a specific disease.

Exceptions: Some pathogens cannot be cultured, some diseases are caused by multiple agents, and some require a human host.

Pathogenicity and Virulence

Definitions

• Pathogenicity: Ability of a microorganism to cause disease.

• Virulence: Degree of pathogenicity.

Virulence Factors

• Adhesion factors

• Biofilms

• Extracellular enzymes

• Toxins

• Antiphagocytic factors

Virulence factors enable pathogens to enter hosts, adhere to cells, access nutrients, and evade the immune system.

Adhesion and Biofilms

• Attachment proteins (adhesins) on microbes bind to host cell receptors.

• Blocking or altering these proteins/receptors can prevent infection.

• Some bacteria form biofilms by attaching to each other or to surfaces via secreted matrices.

The Stages of Infectious Disease

Stages

1. Incubation period: Time between infection and first symptoms.

2. Prodromal period: Mild, general symptoms.

3. Illness: Most severe stage; immune system not fully responded.

4. Decline: Pathogen is being eliminated; symptoms subside.

5. Convalescence: Recovery and tissue repair.

Individuals can be infectious during any of these stages.

Transmission of Infectious Diseases

Modes of Transmission

• Contact transmission: Direct, indirect (via fomites), or droplet.

• Vehicle transmission: Airborne, waterborne, or foodborne.

• Vector transmission: Biological (e.g., mosquitoes) or mechanical (e.g., flies).

Contact Transmission

• Direct contact: Physical contact (e.g., touching, sexual contact, mother to fetus).

• Indirect contact: Via fomites (inanimate objects like toys, needles, money).

• Droplet transmission: Via mucus droplets (e.g., sneezing, coughing). If droplets travel more than 1 meter, considered airborne.

Vehicle Transmission

• Airborne: Pathogens carried through the air (e.g., fungal spores, viruses).

• Waterborne: Pathogens in contaminated water (e.g., cholera, dysentery). John Snow traced cholera outbreaks to water sources.

• Foodborne: Pathogens in undercooked or improperly handled food.

• Bodily fluids: Transmission via blood, saliva, or other fluids (important in healthcare settings).

Vector Transmission

• Biological vectors: Animals that harbor and transmit pathogens (e.g., mosquitoes, ticks).

• Mechanical vectors: Animals that carry pathogens on their bodies (e.g., flies, cockroaches).

Epidemiology

Principles of Epidemiology

• Epidemiology: Study of where and when diseases occur and how they are transmitted.

• Track disease occurrence using two main measures:

Measure	Definition
Incidence	Number of new cases in a given area during a given period
Prevalence	Total number of cases (new and existing) in a given area during a given period

• Occurrence is also evaluated by frequency and geographic distribution.

Patterns of Disease Occurrence

Term	Description
Endemic	Constantly present in a population or region at a relatively stable rate
Epidemic	Sudden increase in cases in a population or region
Pandemic	Sudden increase in cases across several continents or worldwide

Public Health and Disease Control

• Understanding reservoirs, transmission, and epidemiology is essential for controlling infectious diseases and protecting public health.

Example: The spread of avian flu in poultry farms demonstrates the challenge of controlling zoonotic diseases with large animal reservoirs.

Additional info: John Snow is considered the father of epidemiology for his work tracing cholera outbreaks to contaminated water sources in London.