1. Let’s start with a story…
In 1796 an English physician, Edward Jenner, attempted a very famous and risky experiment. In those days people used to die of a deadly disease called smallpox, and there was no cure for it. Jenner noticed that people who worked in farms with cows, almost never contracted the deadly disease smallpox. They however would get sick with a milder disease called cowpox, which somehow protected them from getting smallpox. Based on this observation, Jenner took an eight year old boy and infected him with cowpox (milder disease). When the boy recovered from the disease, he now infected the boy with the deadly disease, smallpox. The boy did not get sick!
Did cowpox protect the boy from smallpox?
How could a previous illness protect a person from getting sick again?