The vaccination stimulates immunity to a disease by exposing the immune system to live, but weakened, organisms or to dead or inactivated organisms. In some modern vaccines genetically engineered analogs of parts of the germ are used to stimulate the nervous system. The immune system responds and creates a molecular memory of the disease organism, which is kept by special white blood cells. In a subsequent invasion by the same germ, the immune system causes these cells to multiply quickly and to produce a defense against the disease. As a result the vaccinated person either remains healthy or has only mild symptoms of the disease.
Vaccination has some risks.
Occasionally individuals develop symptoms related to the vaccination. Most common are allergic reactions to the medium in which the microorganisms are grown. People with problems of the immune system, such as AIDS , may not be able to have vaccinations made with live organisms. Effective vaccinations have been developed to protect against many diseases, including smallpox , measles , mumps , chicken pox , tetanus , rubella , polio and diphtheria . Some vaccinations are given in a single dose, either orally or by injection, while others require multiple doses over a prescribed time period to provide maximum benefit.