Vaccines

A vaccine is a biological preparation that confers active acquired immunity against a specific disease, typically made from weakened (attenuated) or killed forms of the causative microbe, its toxins, or an isolated surface protein. By delivering only a small, harmless amount of the pathogen or its parts, a vaccine lets the immune system learn to recognise the organism and build memory without the recipient ever experiencing the actual disease; because the resulting immune response can fade, some vaccines require repeat 'booster' doses to keep the memory response strong enough to defeat a real future infection.

Vaccines are grouped into three generations by how they are made. First-generation vaccines are further divided into three types: live attenuated vaccines, which use a weakened, less virulent form of the live pathogen (e.g. the MMR — measles, mumps, rubella — vaccine, and the varicella/chickenpox vaccine); killed (inactivated) vaccines, made by killing the pathogen with heat or chemicals (e.g. Salk's polio vaccine); and toxoid vaccines, which use an inactivated version of the pathogen's toxin so the body builds immunity to the toxin's harmful effects rather than to the infection itself (e.g. the DPT vaccine, covering diphtheria, pertussis and tetanus). Second-generation vaccines contain only the pathogen's purified surface antigen rather than …