Vaccination is a way of causing immunity to pathogens like viruses and bacteria by introducing live, killed, or changed antigens into the body. The term comes from the Latin word for cow - vacca - because the first vaccinations used cowpox to induce immunity to smallpox.
How it Works
The vaccine containing the antigens is introduced into the body, usually through injection or ingestion. The antigens stimulate the body to produce antibodies against the disease or a more dangerous form of the disease, working with the immune system's natural ability to ability to destroy proteins that it identifies as foreign or "non-self".
Today most public health experts recommend that children receive vaccination against ten diseases that are particularly dangerous for youth: diphtheria, tetanus or lockjaw, pertussis or whooping cough, polio, hepatitis B, measles, mumps, rubella, Haemophilus influenza b, and chicken pox. Children and adults may require booster shots for some of these as they age. And note that some of the vaccines are given in combined doses, so little kids don't have to endure ten separate pricks; in addition, researchers are trying to develop a vaccine that could give immunity to all these diseases from a single dose. Wouldn't that be nice.
If you're travelling, you might find that other vaccinations are recommended - for yellow fever, for example. The World Health Organization has a list of current recommendations.
Note too that some people oppose immunization on religious grounds - notably Christian scientists; others consider compulsary vaccination a violation of civil liberties. Accordingly, many jurisdictions allow for "conscientious objectors" to refuse vaccination.
History of Vaccination
It has been known since ancient Greek times that those who survived a plague are often immune to reinfection - Thucydides noted this in 429 BC. But it took the enterprising Chinese to put this knowledge into action.
In the 10th century Chinese doctors took infected matter from smallpox lesions and inserted it under the skin of the healthy, or put powdered scabs up the nose (yuck), a practice which caused mild illness in most. The process, which also became known as variolation, did kill a few, but those who survived had immunity to smallpox. And since smallpox was a much-feared disease that killed a fifth to a quarter of those who contracted it, and left fearsome scarring and often blindness in survivors, many chose to take the risk.
By the 18th century the practice had spread to Turkey, where it came to the attention of the wife of the British ambassador, Mary Wortley Montagu; she had her children inoculated, writing to a friend back home that she was "patriot enough to take pains to bring this useful invention into fashion in England". In 1721, during an epidemic, Sir Hans Sloane, royal physician, heard about variolation and practiced it on some prisoners; he found it to be successful, and so the practice gained in popularity.
In the late 18th century Edward Jenner underwent variolation; he later grew up to be a doctor. His patients, rural people who often had cowpox, told him that if they had had a bout of that relatively mild disease they would not catch smallpox, and he observed this to be true. So in 1796 he infected a local boy, James Phipps, with cowpox; when Phipps recovered Jenner injected pus from a smallpox pustule under his skin, but the boy did not get sick. Thus Jenner confirmed in a scientific - if perhaps unethical - manner what folk wisdom had long said to be true. And Jenner's method, which he called vaccination, was preferable to variolation because it caused a much less severe reaction on the skin.
Jenner received funding from the government to continue his studies, and the practice of vaccination became popular. Thomas Jefferson was an enthusiastic supporter of vaccination. But there were opponents as well. Many echoed the sentiments of one George Gibbs, who in 1870 questioned how "a loathsome virus derived from the blood of a diseased brute" could prevent smallpox. The move by governments to make vaccination compulsary was seen by people like Gibbs as a gross violation of civil liberties; he opposed the British act of 1853 legislating innoculation as akin to "spies forcing their way into the family circle". Partly in response to these criticisms, by 1898 the act was revised to allow refusal.
It wasn't till Louis Pasteur's discovery of microorganisms that the mechanism by which innoculation worked was understood. Pasteur also discovered that if the microorganisms were left to grow in a bacterial culture until they grew weak and attenuated, they would still give immunity but not cause disease. He used his method to devise a vaccination against rabies in 1885; today a number of modern vaccines rely on this method.
Emil von Behring and Shibasaburo Kitasato discovered diptheria and tetanus toxins in 1890; they showed that animals injected with the tetanus toxin developed immunity to the disease. Over the next thirty years scientists developed vaccines for diptheria, tetanus, whooping cough, and tuberculosis; after World War II polio and measles vaccines became available.
Availability did not mean use, however. The World Health Organization drove the widespread vaccination of children through their mass immunization campaigns. Their most notable success has been against smallpox, officially declared eradicated in 1980, three years after the last naturally-occuring infection with that dread disease. Although organisms which live outside the human body, like tetanus, can probably never be eradicated, WHO is focusing now on wiping out polio by 2005.