Far more people have cells that briefly behave in cancerous ways then ever actually develop cancer. Most of the time, those cancerous cells are destroyed before they can do any real damage, and scientists can see evidence of this by looking at echoes of past battles with the immune system. If you've had chicken pox, some of the immune cells that fought off that disease will stick around, ready to more-quickly mount a response against a repeat attack. The same seems to be true with certain kinds of cancers. Scientists found immune cells in healthy people that appear to be primed to attack leukemia — leftover remnants of the body's previous, successful skirmishes.