The following report provides evidence that viruses enslaved by ancient animals permitted mammals to evolve. That's us. Viruses are our friends. A gene we borrowed from them is essential for placentas to form properly, and, therefore, for us to successfully make babies.

http://www.nytimes.com/2010/01/12/s
cience/12paleo.html?pagewanted=1

What's your favorite virus anecdote?