Narrator: This is Science Today. Viruses are very tricky and exist for one purpose only - to reproduce. To do that without being foiled by the immune system, they have rapidly evolving evasive strategies. But it seems the immune system can be just as tricky in their counter-attacks. Dr. Lewis Lanier of the University of California, San Francisco, discovered the first genetic evidence of the embattled immune system's attack against viral invaders.

Lanier: These viruses we've come to find out will make essentially decoys or proteins that will turn the immune system off. What does the immune system do to counteract that?

Narrator: Lanier found in the lab that the immune proteins that were manipulated by masquerading invaders were able to evolve by mutating to the point where the immune cells could recognize the virus and create a new receptor to prevent attacking itself.

Lanier: So it has no chance of being deleterious and causing active attack against self. We think this is just the first example of a common event in the immune system where you have this back-and-forth between pathogens and a host. It's an ongoing, constant battle.

Narrator: For Science Today, I'm Larissa Branin.