Cowpox is a mild viral disease that cows can get, causing small blisters on their udders. For centuries, it was common among dairy cows in Europe, and milkmaids would sometimes catch it from touching infected cows. When people got cowpox, they'd develop a few small sores on their hands, feel slightly sick for a few days, then recover completely.

What made cowpox remarkable wasn't the disease itself but what it prevented. In the 1700s, an English doctor named Edward Jenner noticed that milkmaids who'd had cowpox never seemed to catch smallpox, a deadly disease that killed millions of people. Jenner realized that having cowpox somehow protected people against smallpox. In 1796, he took fluid from a milkmaid's cowpox sore and used it to help protect a young boy against smallpox. This experiment led to the world's first vaccine.

Today, cowpox is rare, and smallpox has been completely eliminated from the world, but this humble cow disease changed medical history forever.