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. Although measles spreads through the air, this receptor, called CD150, wasn’t primarily found in nose and lung mucous membrane cells. It was somewhere quite a bit more insidious: immune cells. “This receptor is especially expressed on memory cells of the immune system,” says de Swart. “That means that the virus predominantly infects, and then kills, memory cells of the immune system.” These are the cells that remember pathogens you’ve encountered in the past and help protect you when you meet them again.

Measles is deadlier than most people think

Doctors and researchers had long observed that measles infections seem to kick off a period of sickliness in children, making them more prone to illness—but exactly why was mysterious for many years. In 1995, researchers noticed that giving kids in developing countries a measles vaccine sharply reduced their risk of death across the board. The vaccine seemed to be saving them not just from death by measles, but from death by other infections as well.

Epidemiologists have found that the effects of measles infection might last for years. In 2015, Dr. Michael Mina, de Swart, and colleagues discovered that measles infections raised children’s risk of death for two to three years after they had apparently recovered. "We were effectively saying measles was associated with as much as 50% of all childhood infectious disease deaths not officially having to do with measles,” says Mina. 

“Ultimately, we proved that initial hypothesis immunologically, showing that measles actually did erase anywhere between 20 and 75% of somebody's immunological memory pool,” Mina says. He suspects that after a measles infection, children have to rebuild their immune systems by encountering infections all over again—a process that can take years.

Will we see a return to the pattern of serious childhood illness? 

The sickly childhoods of children’s literature likely reflect a number of factors. Many tales predate the invention of antibiotics—not used for measles, but for many other childhood infections—as well as many vaccines, and up until World War I, they also coincide with a societal fascination with convalescence generally, says Hosanna Krienke, a professor at the University of Wyoming who has written about the literature of long illness. “This kind of slow, delayed recovery is across all of the literature of the time period,” she says. 

It was also often the reality of catching a disease like measles back then—a fact that should alarm parents, says Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia. “If we continue along our current path, where more and more parents are choosing not to vaccinate their children, this phenomenon will become even more evident,” he wrote in an email. “Tough times ahead.”