Perfected Through Infection

After I got my swine flu shot, I was reading up on viruses, and was shocked to find out that we're all part virus:

Endogenous retroviruses (ERVs) are retroviruses derived from ancient viral infections of germ cells in humans, mammals and other vertebrates; as such their proviruses are passed on to the next generation and now remain in the genome.
...
They play a key role in evolution.

So we're not just descended from our "ancestors," we're also descended from some viruses that infected egg cells or sperm cells. And some of the modifications were useful.

Ancestors, I thank you all,
for bringing about my existence,
including those bugs, microscopically small,
who sneaked past your bodies' resistance.