Scientists Crack A 50-Year-Old Mystery About The Measles Vaccine : Goats and Soda : NPRWell, say you get the chicken pox when you're 4 years old. Your immune system figures out how to fight it. So you don't get it again. But if you get measles when you're 5 years old, it could wipe out the memory of how to beat back the chicken pox. It's like the immune system has amnesia, Mina says. "The immune system kind of comes back. The only problem is that it has forgotten what it once knew," he says. So after an infection, a child's immune system has to almost start over, rebuilding its immune protection against diseases it has already seen before. This idea of "immune amnesia" is still just a hypothesis and needs more testing, says epidemiologist William Moss, who has studied the measles vaccine for more than a decade at Johns Hopkins University.