What is herd immunity and why is it important?

Herd immunity (community immunity) occurs when a sufficient proportion of a population is immune to an infectious disease, reducing its spread and protecting those who cannot be vaccinated. The threshold depends on disease transmissibility (R0): measles (R0~15) requires ~95% coverage, while less contagious diseases need lower thresholds. Importance includes: protecting vulnerable individuals who cannot be vaccinated (infants, immunocompromised, elderly); preventing outbreaks; potentially leading to disease elimination/eradication (smallpox); and reducing healthcare burden. Factors affecting herd immunity include vaccine efficacy, duration of immunity, population mixing patterns, and variant emergence. Achieving herd immunity through natural infection is undesirable due to associated morbidity and mortality.