Many people in the U.S. today have no direct experience with measles, a disease that used to infect three or four million people in the nation every year before vaccines became available in 1963. Adults may remember they were vaccinated against the disease in childhood or have only a vague memory the disease is associated with a spotty red rash.
But the virus isn’t entirely gone: The U.S. has already seen nearly 400 confirmed cases of measles in 2025 to date—more than the total in any year since 2019. “We don’t see these outbreaks often, but when we do, they do make headlines,” says Gabriel Benavidez, an epidemiologist at Baylor University in Texas.
With cases on the rise in an outbreak that started in Texas, here are some key things to know about measles.
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Measles Makes You More Vulnerable to Other Infections
Measles is a nasty virus. It takes a week or two for symptoms to develop, giving the highly contagious virus plenty of time to spread to other people. Once an infection sets in, measles typically causes a high fever accompanied by a cough, runny nose, and red, watery eyes. The characteristic rash begins several days later, and the disease can trigger ear infections and diarrhea as well. And like other respiratory viruses, the initial infection can leave people vulnerable to pneumonia.
But that’s not all. A common consequence of a measles infection on the immune system is called immune amnesia. The virus appears to attack B cells, which the immune system uses to remember pathogens it has seen before and respond quickly. One key study published in 2019 found that measles can destroy both B cells trained on nonmeasles infections and so-called naive B cells that respond to future infections. Limited evidence suggests that this reduced immunity can last for a couple years.
Measles Can Cause Brain Inflammation
A rarer complication of measles that occurs in only about one in every 1,000 cases is brain inflammation, or encephalitis. This appears to occur when the measles virus triggers the immune system to attack a protein produced by certain brain cells. The resulting inflammation kills about one in five people.
Even when someone appears to recover fully from measles, a serious risk…
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