You can divide all living creatures into two camps. We humans are in one camp, along with lots of other things like dogs and birds and trees and caterpillars. In the other camp are the things that live inside of us, the bacterias and viruses, the worms and protozoans, in short, parasites. Scientists estimate that the parasites outnumber us and our free-living allies by 3 to 1. Carl Zimmer, author of the book Parasite Rex, talks with Ira about how parasites manage the trick of living inside of us, behind enemy lines, without us finding out. (11 minutes)