Vaccines are preventive medicines which when introduced into the body protect it from a specific disease-causing agent. These are usually derived from the disease-causing agent itself which may be a microbe like bacteria or virus, or a toxic microbial product called toxoid.
Vaccines stimulate the immune system of the body to produce neutralizing substances called antibodies as soon as the disease-causing agent enters the body. Vaccines that protect against a microbial disease usually contain the live but weakened or the dead microbe itself. Vaccines for some viral diseases are also prepared from parts of the disease-causing viruses. For diseases such as tetanus or cholera which are caused by toxoids, vaccines are made from chemically inactivated toxoids. Vaccines also contain preservative chemicals and substances called adjuvants which make the immune system react faster and more strongly. Familiar vaccines such as DPT given to children are special preparations for protecting against several diseases simultaneously. DPT protects against diphtheria, pertussis (whooping cough) and tetanus.