Epidemic

An epidemic is a sudden increase in the number of cases of an illness in a specific population within a region over a defined period, exceeding the expected level based on historical data. The term is commonly used when the rise is localized to a community, city, or country, though it can apply to larger areas if warranted by transmission patterns.

Causes include infectious agents such as viruses and bacteria, as well as environmental factors or changes in host susceptibility. Transmission may occur through person-to-person contact, vectors, contaminated food or water, or other routes depending on the pathogen involved. Epidemics can result from seasonal patterns, shifts in population immunity, or changes in behavior and exposure.

Examples include influenza epidemics and cholera outbreaks that affect particular regions or populations. If a pathogen spreads across multiple countries and continents and affects a large proportion of people, the event is more likely to be described as a pandemic, whereas smaller, localized occurrences may be labeled outbreaks or epidemics.

Surveillance and data collection are used to detect unusual increases in illness. Public health authorities monitor incidence, hospitalizations, and laboratory confirmations, and metrics such as the effective reproduction number (Re) above 1 indicate ongoing transmission and growth of cases.

Response measures focus on rapid diagnosis and case management, isolation where appropriate, contact tracing, vaccination campaigns when available, and infection prevention in healthcare settings. Additional interventions may include public information campaigns, social or travel measures, and efforts to address underlying factors such as vaccination coverage and sanitation.

Terminology noting, endemic means a disease present at a baseline level in a population; an epidemic denotes excess incidence; a pandemic indicates global reach.