Not so long ago, coming down with measles was seen almost as a rite of passage. Before measles vaccination began in the U.S. in the early 1960s, millions of Americans, many of them children, contracted the virus each year—forcing them to weather a flu-like illness and telltale skin rash, but also bestowing lifelong immunity. As a result, some Americans still view measles as relatively harmless—which, in addition to a dangerous uprising of anti-vaccine sentiment, has led some parents to decline shots for their children, contributing to a resurgence of preventable illness in the U.S. and overseas.

A pair of related studies published in Science and Science Immunology, however, busts the myth that measles isn’t dangerous. In addition to being a serious disease in its own right, measles can also virtually wipe out a person’s immune system, leaving them with “immune amnesia” that makes them more susceptible to other diseases, according to the research.

Doctors have long known that measles predisposes sufferers to other illnesses. Measles can lead to serious complications like neurological damage, but many of the approximately 110,000 global measles-related deaths each year actually come from concurrent infections like pneumonia. The new studies are among the first to demonstrate why that happens.

“Every time we see a pathogen, our immune system recognizes this pathogen, builds immunity to it and then stores it in the form of immune memory,” explains Velislava Petrova, a postdoctoral fellow in immunogenetics at the Wellcome Sanger Institute in the U.K. and first author of the report published in Science Immunology. The measles virus, however, seems to attack these memory cells, effectively leaving sufferers with an immune system that no longer remembers the pathogens to which it has already built up immunity, and thus an impaired ability to fight them off.

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Petrova and her colleagues conducted their research in a part of the Netherlands with very low measles vaccination rates. They analyzed blood samples from a group of 26 children ages 4 to 17 who were unvaccinated and had never had measles—meaning they could develop the infection organically—both when they were healthy, and again after a measles outbreak in the community. They also used three unvaccinated children who did not develop measles as a control group.

Blood sample testing revealed the children who had recovered from measles had the right number of white blood cells, crucial to mounting an immune response and fighting off disease. But sequencing revealed that the types of white blood cells weren’t right. “Our immune cells recover back to normal numbers [after getting measles],” Petrova says, “but they are no longer the same memory cells.”

In the related Science study, researchers analyzed the kids’ antibody activity before and after measles infection, and found that, two months after recovery, they had lost up to 73% of their antibody diversity.

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Not only did the virus wipe out memory cells, it also replaced them with new cells that provide immunity against future measles infection, Petrova says. So, while people who come down with measles are protected from future bouts of that virus, they seem to be left unprotected from other, previously known pathogens and ill-equipped to respond to new ones.

The researchers confirmed that finding by infecting flu-vaccinated ferrets with a measles-like illness. After suffering from measles, the animals no longer had immunity against the flu, and experienced more severe flu symptoms, compared to animals that had the flu before contracting measles.

Measles “makes our immune system more baby-like,” Petrova says. “Babies are more vulnerable to infections because their immune system is still maturing. That’s what measles does.”

Future research, Petrova says, will focus on learning how, exactly, measles—”an immunological paradox”—manages this feat. The answer may lie in the virus’ ability to infiltrate and alter bone marrow, the body’s reservoir for immune cells, she says.

More research may be needed, but Petrova emphasizes that there’s plenty of evidence to support measles vaccination now. “Measles is not as harmless of a disease as many people think. The disease itself is a dangerous,” she says. “But what this study shows is vaccination is really important not only to protect us from the disease itself, but also to protect us from other diseases.”