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A vaccine (named after vaccinia[?], the infectious agent of smallpox) is used to prepare a human or animal's immune system to defend the body against a specific pathogen, usually a bacterium, a virus or a toxin. Depending on the infectious agent to prepare against, the vaccine can be a weakened bacterium or virus that lost its virulence[?], or a toxoid[?] (a modified, weakened toxin or particle from the infectious agent).

The immune system recognizes the vaccine particles as foreign, destroys them and "remembers" them. When the virulent version of the agent comes along, the immune system is prepared for a fast strike, neutralizing the agent before it can spread and multiply to vast numbers. Mention immune memory cells and related stuff here

Live but weakened vaccines are used against tuberculosis, rabies, and smallpox; dead agents are used against cholera and typhoid; toxoids against diphtheria and tetanus.

Many vaccines, though they are by far not as virulent as the "real" agent, have unpleasant side effects on the body, and have to be renewed every few years. A new attempt to avoid these obstacles of "classic" vaccination is DNA vaccination. The DNA coding for a part of a virus or a bacterium that is recognizable by the immune system is inserted and expressed[?] in human/animal cells. These cells now produce the toxoid for the infectious agent, without the effects other parts of a weakened agent might have. As of 2001, DNA vaccination is still experimental, but shows some promising results.

Many diseases such as polio have been largely controlled in developed countries through mass use of vaccines (indeed smallpox appears to have been completely eliminated in the wild).

Some people, particularly those who practice alternative medicine, refuse to immunize themselves or their children, as they believe vaccines' side effects outweigh their benefits, much to the chagrin of doctors. However, as long as the vast majority of people are vaccinated it is difficult for an outbreak of disease to spread. This effect is called herd immunity. Advocates of conventional medicine argue that side effects of approved vaccines, whilst real, are either far, far less serious than actually catching the disease, or are very rare.

See also : immunology -- immunization[?] -- medicine -- genetics -- AIDS vaccine

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