Vaccine: Weak or dead pathogens that are injected into a person and will cause the immune system to create antibodies without the person have to experience illness. Should the live pathogen enter the body, the immune systems will immediately attack it.
In the late 18th century a man by the name of Edward Jenner was experiencing the devistating effects of a smallpox epidemic. All around the village/town people were dying of smallpox (like chickenpox but more severe and found both outside and inside the body). He noticed that one particular group of people were not being affected by smallpox- the milk maids; girls about 14 who would go out and milk the cows.
What the milkmaids DID experience was cowpox- a mild form- which they caught from the cows and quickly recoved from in a matter of days. Jenner hypothesized that the disease cowpox was somehow giving the girls the ability to avoid getting smallpox. In that light, he designed a (cowardly) test. Jenner scraped the puss from a cowpox lesion and scratched it into a 5 year old boy- his son- who soon developed cowpox. He recovered, as the milkmaids did, and after a few weeks Jenner then infected the 5 year old with smallpox!
Should his hypothesis be wrong the 5 year old would surely have died. Luckily, Jenner was correct and the boy never contracted smallpox. Soon, everyone was being inoculated with cowpox and recovering. The incidence for smallpox dropped dramatically!
Flash forward to the early 20th century (1930s or so) and we meet Jonas Salk. Up until this time, people had to suffer a disease to gain immunity to it. Polio was a devastating disease of the time that left its survivors paralyzed or worse. Dr. Salk wondered if a person could gain immunity without getting sick. His hypothesis was that dead polio virus would cause the person's immune system to react the same way live polio would and antibodies would be created to fight it off.
In his experiment, Salk injected himself with dead polio virus and waited a few weeks for antibodies to be created (he hoped!). He then injected himself with live polio virus! It worked! Salk became immune to polio without experiencing the terrible affects of having polio. Millions of school children were then vaccinated against polio and the incidence of polio droped from high double digit percentages to single digit percentages. Since that time, children around the world are vaccinated against such stable diseases as mumps, chickenpox, and rubella. Other disease that change or evolve more often have vaccines that will only work for a short period of time- like the flu.