Surviving an infection often confers protection against the next encounter. Immunity is the state of being protected against a particular infectious disease, either through prior infection, vaccination, or transfer of antibodies.

Active immunity arises when the host produces its own antibodies and memory cells; it is long-lasting. Passive immunity is borrowed—across the placenta, in breast milk, or by injection of antibodies—and wanes as those antibodies clear. Herd immunity appears when enough individuals are immune that transmission chains break.

Immunity is rarely absolute; reinfection can occur with high inoculum or drifted strains. Measuring antibody levels or cellular responses helps assess protection in research and some clinical settings. Public-health programs aim to raise population immunity to control or eliminate targeted diseases.

• Protection against a specific infectious disease
• Acquired actively by infection or vaccine, or passively by antibodies
• Supports herd protection when widespread
• Usually durable but not always complete