Why Not Everyone Gets Sick After Exposure: Immunity, Dose, and Chance

Seven Factors Determining Infection Outcome

Host Immune Status: Innate (barriers, phagocytes, NK, complement) + adaptive (memory T/B cells, antibodies). Prior infection/vaccination → faster, stronger response.
Inoculum Size (Dose): Minimum infectious dose (MID) varies by pathogen (e.g., Shigella ~10–100 organisms; Salmonella ~10⁵–10⁷). Below threshold → clearance without disease.
Exposure Intensity & Duration: Close, prolonged contact (household, healthcare) > brief, distant.
Route of Transmission: Airborne (measles, TB) > droplet > contact > vector. Mucosal portals differ in susceptibility.
Pathogen Virulence & Immune Evasion: Capsules, toxins, antigenic variation, intracellular niche, biofilm.
Host Genetic & Physiologic Factors: HLA type, blood group (O protective vs. cholera), CCR5Δ32 (HIV), sickle trait (malaria), age, nutrition, microbiome, comorbidities.
Environmental Modulators: Humidity, temperature, UV, ventilation, sanitation affect pathogen survival/transmission.

Explains variable attack rates in outbreaks.
Supports layered protection: vaccination, ventilation, masks, hygiene, distancing.
Guides post-exposure prophylaxis (PEP) decisions.