A substance that stimulates the body’s immune response to a specific disease is called a vaccine. It presents the immune system with a harmless form, component, or genetic instruction related to a disease-causing organism, helping the body prepare for a later encounter.
Vaccines may use weakened or inactivated microorganisms, purified surface proteins, inactivated toxins, or newer genetic approaches such as messenger RNA. The immune system recognizes the vaccine material as foreign and develops immune memory, including antibodies and specialized cells. If the real pathogen appears later, the response can be faster and stronger.
The word vaccine comes from Variolae vaccinae, Edward Jenner’s term for cowpox, which he used in his 1798 work on protection against smallpox. Vaccination is the act of administering a vaccine; the two terms are related but not interchangeable. Vaccines may prevent infection entirely, reduce transmission, or mainly lessen the severity of disease, depending on the vaccine and pathogen.