Measles makes your immune system forget what it’s defeated before.

Nature reveals research in Science and Science Immunology that describes a one hidden risk of measles – that by catching it, you’ll also become more vulnerable to diseases you’ve already fought off in the past:

The studies highlight the importance of measles vaccinations, says Michael Mina, an infectious-disease immunologist at the Harvard T. H. Chan School of Public Health in Boston, Massachusetts, and a co-author of the Science paper.

When people get an infection, their immune system creates antibodies to fight it off. Once the body clears the infection, special immune cells remember that pathogen and help to mount a faster defence if the virus or bacterium invades again.

The Science study is the first to show definitive evidence that measles can destroy this immune memory, Mina says.

Mina and his colleagues analysed blood samples from 77 unvaccinated children from 3 schools in the Netherlands, taken before and after a measles outbreak in 2013. The team also collected blood samples from 33 children before and after their first vaccination against measles, mumps and rubella (MMR). The researchers analysed the kids’ antibodies using a test that measures the amount, and the strength, of antibodies against thousands of viral and bacterial substances.

Two months after the unvaccinated children recovered from measles, the team found that the virus had erased 11–73% of their antibodies against other bacteria and viruses. Although the reasons behind the high variability in antibody reduction are unclear, the finding shows that the virus alters previously acquired immune memory, Mina says. The kids who received the MMR vaccine showed no reduction in these antibodies.

Mina and his team also infected macaques with measles and monitored the animals’ antibodies against other pathogens for five months. The monkeys lost 40–60% of their antibodies against previously-encountered pathogens, suggesting that the measles virus destroys otherwise-long-lived plasma cells in the bone marrow that can produce pathogen-specific antibodies for decades, Mina says.

Measles also seems to wipe out immune cells that ‘remember’ encounters with specific bacteria and viruses, according to a separate, independent team that published the Science Immunology study.