A vaccine is a biological substance that creates immunity against a specific infectious disease. The vaccine contains a weakened, killed, or modified version of the disease-causing agent or parts of it, which is administered to a person to stimulate their immune system to respond and produce antibodies. These antibodies help to identify and fight the disease-causing agent if the person becomes infected with it.

There are different types of vaccines, including:

1. Live attenuated vaccines – These contain a weakened form of the pathogen that causes the disease.

2. Inactivated vaccines – These contain killed pathogens, so they cannot cause the disease, but can still stimulate the immune system.

3. Subunit, recombinant, or conjugate vaccines – These contain a piece of the pathogen, such as a protein or sugar, that can stimulate an immune response.

4. mRNA vaccines – These use a small piece of messenger RNA to create a protein of the pathogen that can stimulate an immune response.

Once someone receives a vaccine, their immune system recognizes the pathogen or part of it as foreign and produces antibodies to destroy it. In the future, if the person encounters the actual disease-causing pathogen, their immune system will immediately recognize it and produce more antibodies to fight it off.

Vaccines have been one of the most effective tools in the prevention and control of infectious diseases, such as polio, measles, and smallpox. They have helped to save millions of lives and reduce the burden of infectious diseases on individuals and societies.