Answer:
These days there is no smallpox in this world. But hundered years ago, small pox epidemics were not at all common. People used to be very afraid of coming near someone suffering from small pox since they were afraid of catching the disease. However, there was one group of people who did not have this fear. The people would provide nursing care for the victims of small pox. These people were Milkmaids. One day a famous physician named Edward Jenner realized that the Milkman who earlier suffered from cowpox did not catch small pox during epidemics. Cowpox is a very mild disease. So he thought that if the virus of cow pox is injected into the person, he will not suffer from Small Pox. So he started giving the injections of cow pox to people as a vaccine.
Cow pox is a very mild disease as compared to Small Pox.
You know that any vaccine is prepared by injecting the weakened form of virus of any particular disease into the person. E.g - For the vaccine of Chicken Pox. The virus of Chicken pox is weakened and injeted into the body of person. So, he does not have any chances of getting chicken pox in future because when the weakened form of virus of chicken pox is injected into our body our immune system is able to fight it. So, next time when the Chicken Pox virus enters our body our immune system recognises it and able to fight it. So we not get that particular disease.
Similarly, Edward Jenner started injecting the cow pox virus because it is a very mild disease that can be easily cured. So the person who had suffered from cow pox did not get small pox.