By 1796 it had been observed that milk maids who had been exposed to cowpox did not succumb to the deadly plague of smallpox that was ravishing both Europe and Britain. From this observation, Edward Jenner was able to construct the world's first successful vaccine. Which of the following statements best fits Jenner's hypothesis that led him to his creation of the vaccine?
a. if exposure to cowpox gives immunity to smallpox in milkmaids, then exposure to cowpox should give immunity to smallpox in other individuals as well.
b. exposure to smallpox always gives immunity to cowpox.
c. cowpox and smallpox are caused by the same microorganism.
d. if milkmaids exposed to cowpox are immune to smallpox, then cowpox and smallpox are the same disease.
e. if exposure to cowpox gives immunity to smallpox in milkmaids, then milkmaids have a natural immunity and their blood should be used to develop a smallpox vaccine.

If exposure to cowpox gives immunity to smallpox in milkmaids, then milkmaids have a natural immunity and their blood should be used to develop a smallpox vaccine.

Explanation:

Edward Jenner, an English physician introduced the smallpox vaccine. He tested his hypothesis (if exposure to cowpox gives immunity to smallpox in milkmaids, then milkmaids have a natural immunity and their blood should be used to develop a smallpox vaccine) by inoculating a small boy with the pus from cowpox blisters on the hands of a milkmaid who had caught cowpox (a virus similar to smallpox, but less dangerous) to create immunity. This connection between cowpox pus and smallpox in humans helped him to create a vaccine that acted as a preventive treatment for smallpox.