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Amidst and following growth in vaccine hesitancy, measles cases are on the rise again in the United States. A recent outbreak in Texas and New Mexico so far involves more than 140 cases, the majority among children and youth, and has caused the death of a child. One of the most contagious human diseases known, measles travels easily through the air, remains alive on surfaces for hours, and can be transmitted for many days before it causes symptoms. Someone with measles will infect 90% of the unprotected people with whom they come into contact, leading to explosive outbreaks if vaccination rates are low. So even though measles is only occasionally deadly – around 2 of every 1000 children infected will die from complications, a small but significant risk – outbreaks can be massive and cause many deaths, especially among the most vulnerable. An outbreak in 2018 killed 140,000 people around the world, mostly children under five. The bright spot here is that the two doses of the MMR (measles, mumps, and rubella) vaccine are safe and very effective at preventing both illness and transmission for life. Why is the MMR vaccine so steadfast, when our COVID and flu shots need regular updates? The answer is, of course, evolution…