Despite their reputation, not all body invaders are harmful. Parasites are essential for maintaining delicate ecosystems. They live in an intimate, usually long-lasting but costly relationship with their hosts, and it's estimated that 40-50 percent of all animal species fall into this group. Unfortunately, only the harmful species to humans appear to be on our radar. Rachel Nuwer explains the phenomenon of parasitic puppet mastery and why a small, growing cadre is beginning to think seriously about the need for targeted parasite conservation before it's too late.