Vaccine

Vaccines are dead or inactivated organisms or purified products derived from them. There are several types of vaccines currently in use. These represent different strategies used to try to reduce risk of illness, while retaining the ability to induce a beneficial immune response. Some vaccines contain killed, but previously virulent, micro-organisms that have been destroyed with chemicals or heat. Examples are the influenza vaccine, cholera vaccine, bubonic plague vaccine, polio vaccine, hepatitis A vaccine, and rabies vaccine. Some vaccines contain live, attenuated microorganisms. Many of these are live viruses that have been cultivated under conditions that disable their virulent properties, or which use closely-related but less dangerous organisms to produce a broad immune response, however some are bacterial in nature. They typically provoke more durable immunological responses and are the preferred type for healthy adults. Examples include the viral diseases yellow fever, measles, rubella, and mumps and the bacterial disease typhoid. The live Mycobacterium tuberculosis vaccine developed by Calmette and Guérin is not made of a contagious strain, but contains a virulently modified strain called "BCG" used to elicit immunogenicity to the vaccine.