In the 18th century, people died from the effects of smallpox. Except for milkmaids, who seemed to be immune. However, they did sometimes have pox-like lesions on their hands. Therefore, an English physician (Edward Jenner) suspected that cowpox might offer protection against the much more dangerous smallpox. So he decided to inject cowpox into people. And that yielded good results: when those people were later exposed to human smallpox, they proved to be effectively protected. Thus the vaccine was born, which owes its name to the cow. After all, vacca is Latin for cow, vacca -> vaccine. The answer is B, the cow.

• For more information: https://pmc.ncbi.nlm.nih.gov/articles/PMC1200696/