Most people recover from measles. But even then, “their immunity to very common infections that they encounter, maybe on a daily basis, is weakened,” says Rik de Swart, a virologist at Erasmus University Medical Center in Rotterdam in the Netherlands who has studied the phenomenon, which is sometimes known as immune amnesia. In some cases, it can take years to get back to normal. What makes measles so dangerous Measles spreads through the air and is highly contagious: Up to 90% of unvaccinated people exposed to someone with measles will get it. It was once common for millions of people— most of them children —to die every year from the disease. In 1963, the first vaccine for the illness was released, and rates in the U.S. plummeted. Measles was declared eliminated in the U.S. in 2000. That same year, biologists in Japan identified a receptor on the surface of cells that the measles virus uses to break in .…