Moo

Do you remember the "Small Pox" days? When people used to have these large icky macules all over their faces? Me neither. That's because we're born after the small pox era. We owe the scientists who developed the small pox vaccine some thanks. So thank you, nerds. But what has this got to do with moo? First, the word vaccine came from the latin word "vacca" which means cow.

Flashback

Way back before the vaccines were developed, people already have ways to evade small pox. There's this technique called variolation. They make you inhale the skin scrapings from a small pox-ed individual and that's supposed to give you immunity from it. Sadly, this technique only works on immunocompetent persons.

Cow and Vaccines. 

Edward Jenner, who was an english doctor, once went to the farm to observe the hot milkmaids. He so loved the milkmaids because they have milky-smooth skins. But then he realized that these milkmaids are small pox free. So he went home, probably masturbated, and started researching. He chanced upon the work of John Fewster which says that cow pox can actually give you immunity to small pox. He met people who failed in developing a small pox vaccine from cow pox skin scrapings. He said "Maybe we need to use the pus". He looked for a cow pox infected person, collected the pus, and tested his newly developed "vaccine" on a small boy. And the rest is history!

Note: This is just my interpretation of the story.

Reference: Virology class

Here's a photo of Christopher Nolan, one of the most critically-acclaimed directors today.