The idea of injecting a healthy person with a disease as defense against more severe illness can be traced back to China around 200 BC. Ancient healers blew smallpox scabs into patients’ noses in order to protect them from the disease. In 1796, the English physician Edward Jenner (1749–1823) took this technique a step further by injecting a boy with cowpox (a relatively harmless disease related to smallpox) and then, after the boy had healed, exposing him to the smallpox virus. The boy remained healthy. Based on this discovery, Jenner coined the word vaccine from the Latin word vacca, or “cow.”

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Although Jenner didn’t understand the science behind it at the time, researchers have since uncovered how immunization works. Vaccines cause the immune system to ready itself for future attacks. Immunizations made from weakened, harmless strains of viruses are introduced into the body, causing the B and T white blood cells to activate and produce memory cells. These cells reproduce in the body for decades, so that when a person again encounters that specific virus, an arsenal of specialized white blood cells is on hand to destroy it.

Currently, the Centers for Disease Control and Prevention advises that all infants and children receive a number of vaccinations. These shots may be a onetime occurrence or require a number of injections. The diphtheria and tetanus immunization, for instance, consists of a series of five inoculations spread over 5 or 6 years. Since booster shots are required to refresh the body’s memory cells every several years, adults need immunizations, too. Because viruses like those that cause the flu constantly mutate and change, a different flu vaccine is needed every year.

ADDITIONAL FACTS

  1. The boy whom Edward Jenner inoculated was the son of a nearby farmer. Jenner asked if he could perform his experiment on the child; surprisingly, the father agreed.
  2. Some parents have questioned whether preservatives and heavy metals, such as thimerosal and mercury, in vaccines can cause autism in young children. Currently, the Centers for Disease Control and Prevention says that there’s no scientific evidence to support this claim.