A vaccines is an antigenic preparation used to produce active immunity to a disease, in order to prevent or ameliorate the effects of infection by any natural or “wild” strain of the organism. The term derives from Edward Jenner’s use of cowpox (”vacca” means cow in Latin), which, when administered to humans, provided them protection against smallpox, which Pasteur and others perpetuated. The process of distributing and administrating vaccines is referred to as vaccination.

Vaccines against cancer are also being investigated; see cancer vaccine. In this case, the antigenic preparation is derived from the cancer cell. There is also research on an HIV vaccine.
Origin of vaccines

Smallpox was the first disease people tried to prevent by purposely inoculating themselves with other types of infections. Inoculation is believed to have started in India or China before 200 BC. Physicians in China immunized patients by picking off pieces from drying pustules of a person suffering from a mild case of smallpox, grinding the scales to a powdery substance, and then inserting the powder into the person’s nose in order for them to be immunized. In 1718, Lady Mary Wortley Montague reported that the Turks have a habit of deliberately inoculating themselves with fluid taken from mild cases of smallpox. Lady Montague inoculated her own children in this manner. In 1796, during the heyday of the smallpox virus in Europe, an English country doctor, Edward Jenner, observed that milkmaids would sometimes catch [cowpox]] from dairy cows and were generally said to not then be able to catch smallpox. Cowpox is a mild relative of the deadly smallpox virus. Building on the foundational practice of inoculation, Jenner took infectious fluid from the hand of milkmaid Sarah Nelmes. He inserted this fluid, by scratching or injection, into the arm of a healthy local eight year old boy, James Phipps. Phipps then showed symptoms of cowpox infection. Forty-eight days later, after Phipps had fully recovered from cowpox, Jenner injected some smallpox-infected matter into Phipps, but Phipps did not later show signs of smallpox infection.

Types of vaccines:

Vaccines may be living, weakened strains of viruses or bacteria which intentionally give rise to inapparent-to-trivial infections. Vaccines may also be killed or inactivated organisms or purified products derived from them.

There are four types of traditional vaccines:

* Inactivated - these are previously virulent micro-organisms that have been killed with chemicals or heat. Examples are vaccines against flu, cholera, bubonic plague, and hepatitis A. Most such vaccines may have incomplete or short-lived immune responses and are likely to require booster shots.
* Live, attenuated - these are live micro-organisms that have been cultivated under conditions which disable their virulent properties. They typically provoke more durable immunological responses and are the preferred type for healthy adults. Examples include yellow fever, measles, rubella, and mumps.
* Toxoids - these are inactivated toxic compounds from micro-organisms in cases where these (rather than the micro-organism itself) causes illness. Examples of toxoid-based vaccines include tetanus and diphtheria.
* Subunit - rather than introducing a whole inactivated or attenuated micro-organism to an immune system, a fragment of it can create an immune response. Characteristic example is the subunit vaccine against HBV that is composed of only the surface proteins of the virus (produced in yeast)

The live tuberculosis vaccine is not the contagious TB strain, but a related strain called “BCG”; it is used in the United States very infrequently.

A number of innovative vaccines are also in development and also in use:

* Conjugate - certain bacteria have polysaccharide outer coats that are poorly immunogenic. By linking these outer coats to proteins (e.g. toxins), the immune system can be led to recognize the polysaccharide as if it were a protein antigen.
* Recombinant Vector - by combining the physiology of one micro-organism and the DNA of the other, immunity can be created against diseases that have complex infection processes
* DNA vaccination - in recent years a new type of vaccine, created from an infectious agent’s DNA called DNA vaccination, has been developed. It works by insertion (and expression, triggering immune system recognition) into human or animal cells, of viral or bacterial DNA. Some cells of the immune system that recognize the proteins expressed and mount an attack against these proteins and cells expressing them. Because these cells live for a very long time, if the pathogen that normally expresses these proteins is encountered at a later time, they will be attacked instantly by the immune system. One advantage of DNA vaccines is that they are very easy to produce and store. As of 2006, DNA vaccination is still experimental, but shows some promising results.