Historically, a vaccine is a pharmaceutical preparation, administered to stimulate active immunity to a specific disease. Most Americans are given their first vaccinations shortly after birth to protect them against such childhood illnesses as diphtheria, tetanus, pertussis (whooping cough), chickenpox, mumps, measles, German measles (rubella), and poliomyelitis. Each year, these early vaccinations help prevent the deaths of thousands of children. Although most people are immunized during childhood, adolescents and adults should keep vaccinations up-to-date by getting booster inoculations to maintain long-term protection against a number of serious illnesses.

The word vaccine is derived from the Latin word for cow, vacca, and the history of vaccines is, not surprisingly, linked to the cow, as we shall see. The ancient Greeks understood that people who survived plague epidemics were resistant when it struck again. And the ancient Chinese first inoculated people with a weakened strain of the smallpox (variola) virus to prevent further infection. The wife of a British ambassador, Lady Mary Wortley-Montague, introduced the practice of "variolation" to Britain in 1721 after observing the procedure performed in Turkey. But it was not until 1796 that the studies of English country doctor Edward Jenner finally set us on the path that led to what we now call a "vaccine." Aware of the variolation procedure, Jenner noticed that milkmaids who had contracted cowpox, a disease of cattle that causes little harm to humans, were immune to smallpox - a frequently fatal disease. He tested his theory regarding cowpox by scratching the skin of a young boy with fluids from the milkmaid's skin sores. A few weeks later he exposed the boy to smallpox, but the boy remained disease free. While such a procedure would be considered unethical today, Jenner was merely controlling the timing of the boy's exposure to smallpox - because in those days everyone was exposed eventually, usually during childhood.

News of Jenner's achievements spread quickly across Europe. However, it was almost 100 years before other vaccines were developed. And the next ones were developed primarily to prevent animal diseases. In 1879 the famous French scientist, Louis Pasteur, discovered that inoculating chickens with a weakened form of the cholera bacillus immunized them against more virulent forms of the disease. Pasteur then applied the same principle to anthrax, a disease that, in the 1870s, had decimated flocks of French sheep, goats, and other livestock. He also developed a vaccine to 
protect dogs against rabies. The vaccine was later used to save a child who had been bitten by a rabid dog. It was the first instance in which an animal served as model for the development of a vaccine for humans. Since Pasteur's experiments in the late 19th century, animal research has been critically important to the development of vaccines for both humans and animals and continues to be vital today.