What are vaccines and how do they work?

The doctrine holding that infectious diseases are caused by the activity of microorganisms within the body is referred to as the germ theory of disease, also called the pathogenic theory of medicine, which states that microorganisms are the cause of many diseases. Although highly controversial when first proposed in the 19th century, it is now a cornerstone of modern and clinical microbiology. Put simply, disease-causing organisms, be they viruses, microbes, or bacteria, attack the body and produce illness. The immune system, if working correctly, prevents illness by destroying disease-causing microorganisms that threaten the body.

Vaccines, from the Latin word "vacca," or cow, trigger one's immune system's infection-fighting ability and memory without exposure to the actual disease-producing germs. Instead, the person is injected with a dead or much weakened (and not dangerous) version of the pathogen. Vaccines stimulate the body's immune system by triggering an immune response; the immune system goes into high gear to destroy the invader. The immunity one develops following vaccination is similar to the immunity acquired from natural infection. For some diseases, several doses of a vaccine (a booster) may be needed for a full immune response. For others, one shot is sufficient.

One's body can become immune to bacteria or viruses by either developing a natural immunity to the disease or by vaccine-induced immunity.

Natural immunity develops after one has been exposed to an organism, and one's immune system develops a defense (from antibodies and memory cells) to prevent one from getting sick again from that particular type of virus or bacterium. Vaccine-induced immunity results after one receives a vaccine, which makes the body think that it is being invaded by a specific organism and the immune system reacts by destroying, the "invader" and preventing it from infecting the person again. The immunity one develops following vaccination is similar to the immunity acquired from natural infection. The goal is the same: to stimulate an immune response without causing disease.

Briefly, the human immune system works because antigens (proteins from the foreign microorganism) stimulate an immune response leading to the synthesis of antibodies (proteins that attack and destroy viral or bacterial particles). "Memory cells" are produced in an immune response, and these cells remain in the blood-stream ready to mount a quick protective immune response against subsequent infections with the particular disease-causing agent.[4] If the infection was to occur again, the memory cells would respond to inactivate the disease-causing agents, and the individual would not likely become sick.

Vaccines traditionally have been classified into three broad categories: live attenuated, whole-killed, and subunit vaccines.

• Live weakened vaccines use live viruses that have been weakened (attenuated). The result is a strong antibody response that establishes lifelong immunity, but live, attenuated vaccines carry the greatest risk because they can mutate to the virulent form at any time, Because the pathogen is alive, it has the potential to multiply within the human body. Examples include vaccines for measles, mumps, and rubella and for chickenpox.