CODY: So, as bugs infect you, they evolve?

STEVE EIGER: Certainly a disease that enters any population, that disease is selected for. There's something special about it that allows it to infect animals or people.

SANDY: You never hear about the ones that aren't special.

SE: Right. They're just dead. The ones that make it have something that allows them to succeed - the others just fail. In terms of change, to see evidence of our biological evolution, you have to look back over a long time span. With diseases, we can see biological evolution over a short time span. Look at things like the common cold, or the AIDS virus, or resistance to antibiotics -

CODY: Influenza.

SE: Influenza. Right. In our lifetimes - in your lifetimes - we've seen lots of evolution in these biological areas.