Infection, Infectious Diseases, and Epidemiology: Structured Study Notes

Infection and Epidemiology

Key Terminology

Understanding the language of infection and epidemiology is essential for microbiology students. The following terms are foundational:

• Pathology: The study of disease.

• Etiology: The study of the cause of a disease.

• Pathogenesis: The development of disease.

• Infection: Colonization of the body by pathogens.

• Disease: An abnormal state in which the body is not functioning normally.

Symbiosis

Symbiosis refers to the interaction between different biological species. There are several types:

• Mutualism: Both organisms benefit. Example: Bees and flowering plants.

• Commensalism: One benefits, the other is unaffected. Example: Barnacles on whales.

• Amensalism: One is harmed, the other is unaffected. Example: Large tree shading smaller plants.

• Parasitism: One benefits (parasite), the other is harmed (host). Example: Ticks feeding on a dog.

Microbiome of Humans

Normal Microbiota

The human body hosts a variety of microorganisms, collectively known as the microbiome. These are classified as:

• Resident microbiota: Permanent residents, mostly commensal.

• Transient microbiota: Temporary residents, unable to persist due to competition, elimination, or changes in the body.

Examples of Resident Microbiota

• Eyes and Skin: Corynebacterium, Micrococcus, Propionibacterium, Staphylococcus, Candida, Malassezia

• Upper Respiratory Tract: Fusobacterium, Haemophilus, Lactobacillus, Moraxella, Staphylococcus, Streptococcus, Veillonella, Candida

• Upper Digestive Tract: Actinomyces, Bacteroides, Corynebacterium, Haemophilus, Lactobacillus, Neisseria, Staphylococcus, Treponema, Entamoeba

• Lower Digestive Tract: Bacteroides, Bifidobacterium, Clostridium, Enterococcus, Escherichia, Fusobacterium, Lactobacillus, Proteus, Shigella, Candida, Entamoeba, Trichomonas

• Female Reproductive System: Actinomyces, Bacteroides, Corynebacterium, Haemophilus, Lactobacillus, Neisseria, Staphylococcus, Treponema, Entamoeba

• Male Reproductive System: Bacteroides, Fusobacterium, Lactobacillus, Mycobacterium, Peptostreptococcus, Staphylococcus, Streptococcus

Establishing Microbiota

Humans are born with a sterile microbiome, which is established during birth and the first months of life.

Opportunistic Pathogens

Normal microbiota can cause disease under certain conditions:

• Introduction into unusual sites (e.g., Escherichia coli causing UTIs).

• Immune suppression.

• Changes in normal microbiota (e.g., after antibiotics).

• Stressful conditions.

Microbial Antagonism

Normal microbiota protect the host by:

• Occupying niches.

• Competing for nutrients.

• Producing acids and bacteriocins.

Probiotics: Live microbes ingested for beneficial effects.

Terminology of Disease

Common Prefixes and Suffixes

• Carcino-: Cancer (e.g., carcinogenic).

• col-, colo-: Colon (e.g., colitis).

• Dermato-: Skin (e.g., dermatitis).

• -emia: Blood (e.g., viremia).

• Endo-: Inside (e.g., endocarditis).

• -gen: Give rise to (e.g., pathogen).

• Hepat-: Liver (e.g., hepatitis).

• Idio-: Unknown (e.g., idiopathic).

• -itis: Inflammation (e.g., meningitis).

• -oma: Tumor (e.g., papilloma).

• -osis: Condition (e.g., toxoplasmosis).

• -patho: Abnormal (e.g., pathology).

• Septi-: Rotting, pathogens (e.g., septicemia).

• Terato-: Defects (e.g., teratogenic).

• Tox-: Poison (e.g., toxin).

Etiology and Causation of Disease

Etiology

Etiology is the study of the cause of disease. Diseases can be:

• Hereditary: Genetic errors (e.g., sickle-cell anemia).

• Congenital: Structural/functional defects present at birth.

• Degenerative: Result from aging (e.g., renal failure).

• Nutritional: Lack of essential nutrients (e.g., rickets).

• Mental: Emotional or psychosomatic (e.g., skin rash).

• Immunological: Immune system dysfunction (e.g., allergies).

• Neoplastic: Abnormal cell growth (e.g., cancer).

• Endocrine: Hormonal imbalances (e.g., Addison’s disease).

Categories of Diseases

• Infectious: Caused by infectious agents (e.g., influenza).

• Iatrogenic: Caused by medical treatment (e.g., surgical error).

• Idiopathic: Unknown cause (e.g., Alzheimer’s disease).

• Healthcare-Associated/Nosocomial: Acquired in healthcare settings (e.g., Pseudomonas infection).

Koch’s Postulates

Principles and Application

Koch’s postulates are a set of criteria used to establish the causative relationship between a microbe and a disease:

1. The microorganism must be present in every case of the disease.

2. The microorganism must be isolated and grown in pure culture.

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

4. The microorganism must be re-isolated from the experimentally infected host and identified as identical to the original isolate.

Exceptions: Some pathogens cannot be cultured, diseases may involve multiple pathogens, and ethical constraints may prevent testing in humans.

Reservoirs of Infectious Diseases

Types of Reservoirs

Reservoirs are sites where pathogens are maintained:

• Animal reservoir (zoonotic diseases): Spread from animals to humans (e.g., rabies).

• Human carriers: Asymptomatic but infectious individuals (e.g., Typhoid Mary).

• Nonliving reservoir: Soil, water, food (e.g., botulism, cholera).

Common Zoonoses

Zoonoses are diseases transmitted from animals to humans. The following table summarizes some common zoonoses:

Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Tapeworm infestation	Dipylidium caninum	Dogs	Ingestion of larvae in dog saliva
Malaria	Plasmodium spp.	Monkeys	Bite of Anopheles mosquito
Ringworm	Trichophyton spp.	Domestic animals	Direct contact
Anthrax	Bacillus anthracis	Domestic livestock	Direct contact, inhalation
Rabies	Rabies virus	Dogs, cats, skunks, bats	Bite of infected animal
Yellow fever	Flavivirus	Monkeys	Bite of Aedes mosquito
Additional info: See full table for more examples.

Classifying Infectious Diseases

Communicable, Contagious, and Noncommunicable Diseases

• Communicable: Spread from host to host (e.g., typhoid fever).

• Contagious: Easily spread (e.g., measles, influenza).

• Noncommunicable: Not transmitted between hosts (e.g., tetanus).

Occurrence of Disease

• Endemic: Constantly present (e.g., common cold).

• Epidemic: Many hosts in a short time (e.g., polio).

• Pandemic: Worldwide epidemic (e.g., AIDS).

• Herd immunity: Immunity in most of a population.

• Sporadic: Occasional occurrence.

Severity or Duration of Disease

• Acute: Rapid onset (e.g., influenza).

• Chronic: Slow development (e.g., tuberculosis).

• Subacute: Intermediate symptoms.

• Latent: Periods of inactivity (e.g., herpes).

Extent of Host Involvement

Types of Infection

• Local infection: Limited to a small area (e.g., boils).

• Systemic infection: Throughout the body (e.g., gangrene).

• Focal infection: Systemic infection originating locally (e.g., tonsils).

• Sepsis: Spread of microbes or toxins.

• Bacteremia: Bacteria in the blood.

• Septicemia: Growth of bacteria in the blood.

• Toxemia: Toxins in the blood.

• Viremia: Viruses in the blood.

• Primary infection: Initial illness.

• Secondary infection: Opportunistic infection after primary.

• Subclinical disease: No noticeable symptoms.

Contamination and Infection

• Contamination: Presence of microbes.

• Infection: Microbes evade defenses, multiply, and establish in the body.

The Stages of a Disease

Progression of Disease

The course of an infectious disease is divided into distinct stages:

• Incubation period: No signs or symptoms.

• Prodromal period: Mild signs or symptoms.

• Period of illness: Most severe signs and symptoms.

• Period of decline: Signs and symptoms subside.

• Period of convalescence: Return to pre-diseased state.

Portals of Entry

Major Pathways

• Skin: Barrier, but can be breached by cuts or burrowing pathogens.

• Mucous membranes: Respiratory tract is most common; gastrointestinal tract must survive stomach acid.

• Placenta: Usually a barrier, but some pathogens can cross.

• Parenteral route: Direct deposition beneath skin or mucous membranes (e.g., cuts).

The Nature of Infectious Disease

• Symptoms: Subjective, felt by patient.

• Signs: Objective, observed by others.

• Syndrome: Group of symptoms and signs.

• Asymptomatic/subclinical: No symptoms, but signs may be present.

Virulence Factors of Infectious Agents

Pathogenicity and Virulence

Pathogenicity is the ability to cause disease; virulence is the degree of pathogenicity. Virulence factors include:

• Adhesion factors: Enable attachment to host cells.

• Extracellular enzymes: Dissolve structural chemicals, aiding invasion.

• Toxins: Harm tissues or trigger damaging immune responses.

• Antiphagocytic factors: Evade host immune system.

The Role of Adhesion

• Attachment proteins (ligands) bind host cell receptors.

• Host cell specificity is determined by ligand-receptor interaction.

• Biofilm formation can enhance adhesion.

Extracellular Enzymes

• Secreted enzymes dissolve host structures.

• Mutants lacking these enzymes are often avirulent.

Toxins

• Exotoxins: Proteins secreted by bacteria.

• Endotoxins: Lipid A of LPS in Gram-negative bacteria.

• Toxemia: Toxins in the bloodstream.

Antiphagocytic Factors

• Prevent phagocytosis (e.g., capsules, antiphagocytic chemicals).

• Leukocidins destroy phagocytic cells.

Incubation Periods of Selected Infectious Diseases

• Staphylococcus foodborne infection: <1 day

• Influenza: ~1 day

• Cholera: 2–3 days

• Genital herpes: ~5 days

• Tetanus: 5–15 days

• Syphilis: 10–21 days

• Hepatitis B: 70–100 days

• AIDS: 1–8+ years

• Leprosy: 10–30+ years

Modes of Infectious Disease Transmission

Transmission Pathways

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

• Vehicle transmission: Airborne (aerosols), waterborne (fecal-oral), foodborne, bodily fluids.

• Vector transmission: Biological (host for pathogen), mechanical (surface carrier).

Disease Classification

Diseases can be classified by taxonomic category, affected body system, longevity/severity, transmission mode, and population effects.

Epidemiology of Infectious Diseases

Frequency of Disease

• Incidence: New cases in a given area/time.

• Prevalence: Total cases in a given area/time.

Epidemiological Studies

• Descriptive: Tabulation of data, identification of index case.

• Analytical: Determining cause, transmission, prevention; often retrospective.

• Experimental: Testing hypotheses, application of Koch’s postulates.

Epidemiology and Public Health

• Data sharing among agencies (CDC, WHO).

• Public health agencies limit transmission via cleanliness, immunization, vector control, isolation, and education.

Hospital Epidemiology: Healthcare-Associated (Nosocomial) Infections

Types of Healthcare-Associated Infections

• Exogenous: Acquired from healthcare environment.

• Endogenous: Arises from patient’s own microbiota.

• Iatrogenic: Results from medical procedures.

• Superinfections: Antimicrobial drugs allow other microbes to thrive.

Control Measures

• Aggressive infection control is required.

• Handwashing is the most effective method.