Infection, Infectious Diseases, and Epidemiology (Chapter 14): Microbiology Study Notes

Infection, Infectious Diseases, and Epidemiology

Introduction

This chapter explores how infectious microbes are acquired, the relationships between microbes and humans, and the principles of epidemiology. Understanding these concepts is essential for recognizing how diseases spread and how they can be controlled.

Types of Symbiosis in the Human Microbiome

Symbiotic Relationships

Symbiosis refers to "living together" and describes the interactions between different organisms, including microbes and humans. The microbiome consists of microbes that colonize the human body without normally causing disease. These are also known as normal microbiota, normal flora, or indigenous microbiota.

• Mutualism: Both organisms benefit. Example: Bacteria in the human gut synthesize vitamins for the host and receive nutrients.

• Commensalism: One organism benefits, the other is neither helped nor harmed. Example: Mites living on human skin.

• Amensalism: One organism is harmed, the other is unaffected. Example: Fungus secreting an antibiotic that kills nearby bacteria.

• Parasitism: One organism benefits at the expense of the other. Example: Pathogenic bacteria causing disease in humans.

Type of Symbiosis	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Neither benefits nor is harmed	Mites on human skin
Amensalism	Is harmed	Neither benefits nor is harmed	Fungus secreting antibiotic
Parasitism	Benefits	Is harmed	Pathogenic bacteria

Reservoirs of Infectious Diseases

Types of Reservoirs

Reservoirs are sources where pathogens are maintained and from which infection can occur. There are three main types:

• Animal Reservoirs: Animals harbor pathogens that can infect humans (zoonoses).

• Human Reservoirs: Infected individuals may actively show symptoms or be carriers without symptoms. Carriers can transmit pathogens before, during, or after illness.

• Nonliving Reservoirs: Soil, water, and food can serve as reservoirs for pathogens. These are often contaminated by feces or urine.

Fomites are nonliving objects (e.g., doorknobs, cell phones) that can transfer pathogens but do not support their growth.

Modes of Disease Transmission

Contact Transmission

Pathogens can be transmitted through various forms of contact:

• Direct Contact: Physical interaction (e.g., touching, kissing).

• Indirect Contact: Transmission via fomites.

• Droplet Transmission: Pathogens spread in droplets expelled by coughing or sneezing.

Vehicle Transmission

Pathogens are carried by contaminated materials:

• Airborne: Spread via dust particles or aerosols (e.g., chickenpox).

• Waterborne: Spread through contaminated water.

• Foodborne: Spread via contaminated food.

Vector Transmission

Vectors are animals (often arthropods) that transmit pathogens:

• Biological Vectors: Pathogen develops or multiplies within the vector (e.g., mosquitoes transmitting malaria).

• Mechanical Vectors: Pathogen is passively carried on the vector's body (e.g., flies transferring bacteria from feces to food).

Portals of Entry and Exit

Entry

Microbes enter the body through specific portals:

• Skin: Intact skin is a barrier; pathogens may enter through cuts or abrasions.

• Mucous Membranes: Respiratory, gastrointestinal, and urogenital tracts are common entry points.

• Placenta: Some pathogens can cross the placenta to infect the fetus.

• Parenteral Route: Pathogens are deposited directly beneath the skin or mucous membranes (e.g., via needles).

Exit

Pathogens leave the host through portals of exit, often the same as entry points, via body secretions or excretions.

Contamination, Infection, and Disease

Definitions

• Contamination: Presence of microbes on or in the body.

• Infection: Successful invasion and colonization of the body by a pathogen.

• Disease: Infection that results in noticeable impairment of body function.

• Carriers: Individuals who harbor pathogens without showing symptoms.

Virulence Factors of Infectious Agents

Pathogenicity and Virulence

Pathogenicity is the ability of a microorganism to cause disease. Virulence is the degree of pathogenicity. Virulence factors are traits that enable pathogens to cause disease.

• Adhesion Factors: Enable pathogens to attach to host cells.

• Biofilm Formation: Microbes form protective communities.

• Extracellular Enzymes: Help pathogens invade tissues.

• Toxins: Chemicals that damage host tissues or trigger immune responses.

• Antiphagocytic Factors: Help pathogens evade destruction by immune cells.

Toxins

• Exotoxins: Secreted by bacteria into the environment; can cause tissue damage.

• Endotoxins: Lipid A component of Gram-negative bacterial cell walls; released when bacteria die. Can cause fever, inflammation, shock, and blood coagulation.

Frequency and Location of Disease

Epidemiological Terms

• Endemic: Disease occurs at a stable frequency within a population.

• Sporadic: Disease occurs irregularly and infrequently.

• Epidemic: Disease occurs at a higher frequency than usual.

• Pandemic: Epidemic that spreads across continents or worldwide.

Incidence and Prevalence

• Incidence: Number of new cases in a specific time and place.

• Prevalence: Total number of cases (old and new) in a population at a given time.

Term	Definition
Incidence	Number of new cases in a specific period
Prevalence	Total number of cases (old and new) at a given time

Example: If there are 2,000 new cases of influenza in a week and 30,000 total cases, the incidence is 2,000 and the prevalence is 30,000.

Additional info: These concepts are foundational for understanding how infectious diseases spread and are monitored in populations.