Why are you immune to most diseases once have been infected or vaccinated?

Once you have been infected with a disease or have been vaccinated, you develop immunity to that particular disease. This immunity is due to the body's immune response, which produces antibodies that recognize and attack the specific pathogen. These antibodies remain in the body and provide protection against future infections by the same pathogen. This phenomenon is called immunological memory.

Natural Immunity:

After an individual recovers from an infectious disease, their immune system retains memory cells that specifically target the pathogen responsible for the infection. These memory cells are a type of white blood cells called B cells and T cells. If the same pathogen is encountered again, the memory cells quickly recognize it and mount an immune response, eliminating the pathogen before it can cause symptoms or establish infection. This is why most people only experience certain diseases once in their lifetime.

Vaccination:

Vaccines work by stimulating the immune system to produce antibodies against specific pathogens without causing the actual disease. When vaccinated, the body recognizes the weakened or inactivated form of the pathogen in the vaccine and produces an immune response, resulting in the formation of memory cells. Should the vaccinated individual encounter the real pathogen in the future, their immune system can swiftly respond and prevent or lessen the severity of the disease.

The success of vaccination programs has been instrumental in controlling and eradicating many infectious diseases, including smallpox, polio, and measles. It is crucial to maintain high vaccination rates to achieve herd immunity, protecting vulnerable populations and preventing outbreaks.