Vaccine

A vaccine is a substance given to provoke an adaptive immune response against a particular pathogen, or in some cases a tumor, without causing the disease itself. It contains or produces an antigen, a molecule the immune system can recognize. Depending on the type, that may be a weakened live microbe, a killed one, a purified piece of one, an inactivated toxin, or genetic instructions in DNA or RNA that let the body's own cells make the antigen for a short time.

The first dose sets off a primary response in which B cells and T cells specific to that antigen multiply, and some become long-lived memory cells. If the person later encounters the real pathogen, these memory cells launch a fast, strong secondary response that often clears the infection before illness develops. Some vaccines need several doses, or later boosters, to build and keep this memory. Protection gained this way is called artificial active immunity.

When enough people in a community are immune, a pathogen struggles to spread, which also shields those who cannot be vaccinated; this is herd immunity. The word comes from the Latin for cow, after Edward Jenner used cowpox material to protect against smallpox in 1796.