16.6 Principles of Disease, Epidemiology & Microbial Pathogenicity

Principles of Disease, Epidemiology & Microbial Pathogenicity

Quick Answer: Epidemiology measures how disease moves through populations (incidence = new cases, prevalence = existing cases); pathogenicity is the capacity to cause disease and virulence is its degree. PA-CAT asks you to read a scenario and pick the correct transmission mode, epidemic type, or virulence factor.

Principles of Disease

A pathogen is a microbe capable of causing disease. Infection is the establishment of the microbe in the host; disease is the resulting damage. The chain of infection has six links: infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host. Breaking any link interrupts spread — the basis of hand hygiene, isolation, and vaccination.

Koch's postulates establish that a specific microbe causes a specific disease: (1) the microbe is found in every case and absent in healthy hosts; (2) it can be isolated and grown in pure culture; (3) it causes disease when inoculated into a susceptible host; (4) it can be re-isolated from the experimental host. Molecular Koch's postulates (Falkow) replace culture with genetic criteria: the gene is found in virulent strains, inactivation attenuates virulence, and reversion restores it.

Epidemiology: Incidence, Prevalence, and Transmission

Incidence = new cases / person-time at risk. Prevalence = existing cases / total population at a point (point prevalence) or period (period prevalence). For a stable disease, prevalence ≈ incidence × average duration.

An outbreak is a sudden increase in cases above baseline in a localized area; an epidemic is an outbreak that has spread widely; a pandemic crosses continents. A common-source outbreak arises from a shared contaminated source (e.g., a single contaminated water supply) and typically produces a sharp curve; a propagated outbreak is person-to-person with staggered case peaks (e.g., influenza).

Transmission modes (Table):

The basic reproduction number R₀ is the expected secondary cases from one infected individual in a fully susceptible population. R₀ > 1 allows spread; herd immunity threshold ≈ 1 − 1/R₀.

Microbial Mechanisms of Pathogenicity

Adherence and colonization: bacteria use pili/fimbriae (e.g., E. coli, Neisseria), adhesins, and biofilms. Invasion: organisms cross epithelial barriers via enzymes such as hyaluronidase (Staph/Strep, spreads through tissue), collagenase, and IgA proteases (N. gonorrhoeae, H. influenzae cleave mucosal IgA).

Capsules (S. pneumoniae, H. influenzae type b, N. meningitidis, Klebsiella) resist phagocytosis; antibody against the capsule is protective and is the basis of conjugate vaccines.

Toxins:

• Exotoxins are secreted proteins. A-B toxins (diphtheria toxin, cholera toxin, tetanus toxin, botulinum toxin) consist of an active A subunit and a binding B subunit. Superantigens (Staph TSST-1, Strep pyrogenic exotoxins) cross-link MHC II and TCR to polyclonal T cell activation and massive cytokine release (toxic shock). Membrane-disrupting toxins (Staph α-toxin, Strep streptolysins) form pores.
• Endotoxin is the lipopolysaccharide (LPS) of Gram-negative outer membranes, released on cell lysis; it binds TLR4/CD14/MD-2, activates macrophages, and drives TNF-α, IL-1, IL-6 release causing fever, hypotension, and DIC — the septic shock cascade.

Immune evasion: antigenic variation (Borrelia relapsing fever, trypanosomes, Neisseria gonorrhoeae pili), intracellular survival (Mycobacteria inhibit phagolysosome fusion; Legionella, Listeria, Salmonella), IgA proteases, and complement resistance (M protein of Strep pyogenes binds factor H).

Virulence vs. Pathogenicity

Pathogenicity is qualitative (can it cause disease?); virulence is quantitative (how severe?). The LD50 (lethal dose for 50% of hosts) and ID50 (infectious dose for 50%) are quantitative measures; a lower LD50 or ID50 indicates higher virulence. Attenuation is the reduction of virulence, the principle behind live-attenuated vaccines.

Nosocomial and Opportunistic Infections

Healthcare-associated infections (HAIs) illustrate how transmission mode shapes prevention. Central line-associated bloodstream infections (often S. aureus, coagulase-negative staphylococci, Candida) are reduced by bundle protocols at insertion. Catheter-associated UTIs (E. coli, enterococci) are prevented by minimizing catheter duration. Ventilator-associated pneumonia (Pseudomonas, Acinetobacter, MRSA) is reduced by head-of-bed elevation and oral care. Surgical site infections are reduced by appropriate perioperative antibiotic prophylaxis and skin preparation. Clostridioides difficile spread is curtailed by contact precautions and antibiotic stewardship. Each intervention maps to a specific link in the chain of infection.

Stages of Infectious Disease

A useful clinical framework traces infection through stages: incubation (entry to first symptoms, varies by pathogen — e.g., influenza 1–4 days, measles 10–12 days, hepatitis B 60–90 days), prodrome (non-specific early symptoms), acute illness (peak signs and symptoms), decline, and convalescence. Some infections add latency (herpesviruses, HIV, TB) or chronic carriage (HBV, HCV, Salmonella typhi). The incubation period is a frequent PA-CAT clue pairing an exposure time with a presenting illness.