A vaccine tricks the body’s immune system into producing antibodies to fight a form of the virus that is not harmful. Then, if the person ever encounters the real and dangerous virus, the body is ready to prevent it from harming any cells.

An Idea in Search of a Method
If everyone in a room suddenly became exposed to the same disease in the same way at the same time, everyone would not be equally affected. One of the most important factors in determining how or whether a person gets sick is immunity—the human body’s own ability to prevent disease.

It has been recognized for centuries that some diseases never reinfect a person after recovery. Smallpox was the first disease people tried to prevent by intentionally inoculating themselves with infected matter. Inoculation originated in India or China some time before 200 BC.

The concept of immunization, or how to artificially induce the body to resist infection, received a big boost in 1796, when physician Edward Jenner inoculated a young boy in England and successfully prevented him from getting smallpox. Jenner used a lancet to scratch some infected material from a woman with cowpox (similar to smallpox) under the boy’s skin.

These smallpox inoculation devices illustrate both the simplicity of the idea and the complexity of the task. Left to right, from upper left corner: three examples of scab protectors (used after inoculation; early 20th century); two types of current disposable devices; bifurcated needles (a significant invention in 1968 because they used less vaccine and could be sterilized and reused); ivory vaccination points in glass carrier with wood shell (1900); vaccinator with metal carrying tube (19th century); spring lancet (1930s); glass and ivory points; round cowpox scab carrier (1860s, to transport vaccinating material); folding vaccinator (early 19th century); trigger vaccinator (1866); ivory-handled lancets with box (18th century); and drum vaccinator (19th century). The photograph shows a man with the distinctive smallpox blisters that often left permanent scars. Hugh Talman, photographer.

Can’t Catch This: Immunity and Immunization
Lack of immunity to disease has helped to decide the fate of entire communities, from smallpox among the Indians in the New World to syphilitic soldiers in the Old. Most people have some amount of natural immunity. The human body can take care of itself in many circumstances—cuts, colds, and minor infections disappear without major upheaval. In other cases, the body has little or no naturally occurring immunity, so if you are exposed to diseases such as polio, influenza, smallpox, hepatitis, diphtheria, measles, or whooping cough, you will probably get sick with it, unless you have been immunized.

Immunization refers to the artificial creation of immunity by deliberately infecting someone so that the body learns to protect itself. An important part of the history of immunization has been determining how to get the immunizing agent into the body. The skin, which keeps germs and mischievous substances out, is also a barrier to getting medicines and vaccines into the tissue where they can work. Physicians have used varying methods to create immunity where there is none.