Chapter 14: Infection, Infectious Diseases, and Epidemiology – Study Notes

Symbiosis

Types of Symbiotic Relationships

Symbiosis refers to the close association between two different species. These relationships can be classified based on the effects on the organisms involved.

• Mutualism: Both organisms benefit from the relationship. Example: Escherichia coli in the human gut synthesizes vitamins for the host and receives nutrients.

• Commensalism: One organism benefits, while the other is neither helped nor harmed. Example: Skin bacteria that feed on dead cells without affecting the host.

• Amensalism: One organism is harmed, while the other is unaffected. Example: Penicillium mold secreting penicillin, which inhibits nearby bacteria.

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

Microbiome of Humans

Types of Microbiota

The human body hosts a diverse community of microorganisms, collectively known as the microbiome.

• Resident Microbiota: Microbes that are a permanent part of the normal microbiome, typically without causing disease.

• Transient Microbiota: Microbes that temporarily colonize the body and are removed by competition, immune system, or physical changes.

• Opportunistic Pathogens: Normally harmless microbes that can cause disease under certain conditions (e.g., immunosuppression, changes in microbiota).

Reservoirs of Infectious Diseases

Three Types of Reservoirs

Reservoirs are sites where pathogens are maintained as a source of infection.

• Animal Reservoirs (Zoonoses): Diseases transmitted from animals to humans. Example: Rabies virus in bats and dogs.

• Human Reservoirs: Infected individuals who may be symptomatic or asymptomatic carriers.

• Nonliving Reservoirs: Soil, water, and food that can harbor pathogens (e.g., Clostridium in soil).

Exposure to Microbes

Contamination vs. Infection

• Contamination: Presence of microbes on or in the body without tissue invasion or damage.

• Infection: Successful invasion and colonization of the host by a pathogen, potentially leading to disease.

Portals of Entry

Major Portals of Entry

Pathogens enter the body through specific routes known as portals of entry.

• Skin: Through cuts, abrasions, or insect bites.

• Mucous Membranes: Lining of respiratory, gastrointestinal, urinary, and reproductive tracts.

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

• Parenteral Route: Direct deposition into tissues via punctures, injections, or wounds.

Role of Adhesion in Infection

Mechanisms of Pathogen Attachment

• Attachment Proteins: Specialized structures (e.g., fimbriae, adhesins) that allow pathogens to bind to host cells.

• Biofilm Formation: Communities of microbes attached to surfaces, enhancing colonization and resistance to host defenses.

Disease and Morbidity

Definitions and Concepts

• Symptoms: Subjective effects of disease felt by the patient (e.g., pain, fatigue).

• Signs: Objective, measurable changes (e.g., fever, rash).

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

• Asymptomatic (Subclinical): Infections that do not produce noticeable symptoms.

Etiology: The Study of Disease Causation

Koch’s Postulates

• 1. The suspected pathogen must be present in every case of the disease.

• 2. The pathogen must be isolated and grown in pure culture.

• 3. The cultured pathogen must cause the disease when introduced into a healthy host.

• 4. The same pathogen must be re-isolated from the experimentally infected host.

Exceptions: Some pathogens cannot be cultured, have no animal model, or cause multiple diseases.

Difficulties: Ethical concerns, polymicrobial diseases, and asymptomatic carriers complicate application.

Pathogenicity and Virulence

Virulence Factors

• Adhesion Factors: Enable attachment to host cells.

• Biofilm: Protects microbes from host defenses and antibiotics.

• Extracellular Enzymes: Degrade host tissues (e.g., hyaluronidase, collagenase).

• Toxins: Poisonous substances that damage host tissues.

  • Toxemia: Presence of toxins in the blood.

  • Exotoxins: Secreted proteins causing specific effects (e.g., botulinum toxin).

  • Endotoxins: Lipid A component of Gram-negative bacterial LPS, causing systemic effects.

• Antiphagocytic Factors: Prevent phagocytosis by host immune cells (e.g., capsules, leukocidins).

Stages of Infectious Disease

Progression of Disease

• Incubation Period: Time between infection and appearance of symptoms.

• Prodromal Period: Short period of mild, nonspecific symptoms.

• Illness: Most severe stage with characteristic signs and symptoms.

• Decline: Symptoms subside as the immune response or treatment reduces pathogen numbers.

• Convalescence: Recovery and return to normal health.

Portals of Exit

How Pathogens Leave the Host

Pathogens exit the body through the same portals as entry, often via secretions or excretions (e.g., respiratory droplets, feces, urine, blood).

Modes of Transmission

How Infectious Agents Spread

• Contact Transmission: Direct (person-to-person), indirect (fomites), or droplet transmission.

• Vehicle Transmission: Spread via air, water, food, or bodily fluids.

  • Airborne (Aerosol): Pathogens carried on dust or droplets in the air.

  • Waterborne: Contaminated water as a vehicle.

  • Foodborne: Ingestion of contaminated food.

  • Bodily Fluid Transmission: Blood, saliva, semen, etc.

• Vector Transmission: Animals that transmit disease between hosts.

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

  • Mechanical Vectors: Passive transport on body parts (e.g., flies carrying bacteria on feet).

Classification of Infectious Diseases

Criteria for Classification

• Taxonomic Categories: Based on causative agent (bacteria, virus, fungus, etc.).

• Body System Affected: Respiratory, digestive, nervous, etc.

• Longevity and Severity: Acute, chronic, subacute, latent.

• Mode of Transmission: Contact, vehicle, vector.

• Effects on Populations: Endemic, epidemic, pandemic, sporadic.

Epidemiology of Infectious Diseases

Measuring Disease Frequency

• Incidence: Number of new cases in a given time period.

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

Occurrence of Disease

Patterns of Disease in Populations

• Endemic: Constantly present in a population.

• Sporadic: Occurs occasionally and irregularly.

• Epidemic: Sudden increase in cases above expected levels.

• Pandemic: Epidemic that spreads across continents or worldwide.

Epidemiological Studies

Types of Studies

• Descriptive: Collects data on occurrence, location, and time of disease cases.

• Analytical: Determines probable cause, mode of transmission, and prevention.

• Experimental: Tests hypotheses about disease causation and prevention.

Hospital Epidemiology

• Exogenous: Pathogen acquired from the healthcare environment.

• Endogenous: Pathogen arises from normal microbiota due to factors within the patient.

• Iatrogenic: Resulting from medical procedures.

• Superinfections: Secondary infections due to disruption of normal microbiota.

Epidemiology and Public Health

Epidemiology informs public health policies and interventions to control and prevent infectious diseases.

Tables

Table: Types of Symbiosis (Table 14.1)

Type	Effect on Host	Effect on Microbe	Example
Mutualism	Benefited	Benefited	E. coli in gut
Commensalism	Unaffected	Benefited	Skin bacteria
Amensalism	Harmed	Unaffected	Penicillium and bacteria
Parasitism	Harmed	Benefited	Pathogenic bacteria

Table: Types of Disease Transmission

Mode	Subtypes	Example
Contact	Direct, Indirect, Droplet	Common cold
Vehicle	Airborne, Waterborne, Foodborne, Bodily fluids	Cholera (waterborne)
Vector	Biological, Mechanical	Malaria (biological), Salmonella (mechanical)

Table: Stages of Infectious Disease

Stage	Description
Incubation	Time between infection and first symptoms
Prodromal	Mild, nonspecific symptoms
Illness	Most severe symptoms
Decline	Symptoms subside
Convalescence	Recovery phase

Table: Occurrence of Disease

Term	Definition
Endemic	Constantly present in a population
Sporadic	Occasional, irregular cases
Epidemic	Sudden increase above normal
Pandemic	Worldwide epidemic

Table: Hospital Epidemiology Terms

Term	Definition
Exogenous	Acquired from environment
Endogenous	Arises from patient’s own microbiota
Iatrogenic	Result of medical procedures
Superinfection	Secondary infection after disruption of normal microbiota