CBSE Class 9 Answered

In 1788, during an epidemic of smallpox, Jenner observed that those of his patients who worked with cattle and had come in contact with the much milder disease called cowpox never came down with smallpox.

Jenner believed that if he could inject someone with cowpox, the germs from the cowpox would make the body able to defend itself against the dangerous smallpox germs which he would inject later.

Jenner then inoculated the liquid from the milkmaid's cowpox sores into the open wounds of a local farmer's boy. The boy went down with cowpox but was not very ill. Six weeks later when the boy had recovered, Jenner vaccinated him again, this time with the smallpox virus, but the boy did not contract small pox. The immunity which he developed during the cow pox protected him from small pox.