Don't have a cow...

Flu vaccine clinics are up and running again. Where does the word “vaccination” come from? In 1796, the British scientist Edward Jenner discovered that immunity to smallpox could be achieved by inoculating people with the milder virus of a disease called cowpox. But, being an 18th-century guy, Jenner gave the virus a Latin name: variolae vaccinae, from the Latin for “cow”, vacca. A cowpox shot was called a “vaccine inoculation”, quickly replaced by the snappier “vaccination”, which was subsequently applied to inoculations against any disease, not just smallpox.
For more on whether "flu" can be used in the plural, and if so, what that plural is, visit this post.