The vast majority of these cases have been children who were not fully vaccinated. Vaccine hesitancy is thought to be a significant reason children are missing out on important vaccines—the World Health Organization described it as one of the 10 leading threats to global health in 2019. And if we’re seeing more measles cases now, we might expect to soon see more cases of other vaccine-preventable infections, including some that can cause liver cancer or meningitis.

Some people will always argue that measles is not a big deal—that infections used to be common, and most people survived them and did just fine. It is true that in most cases kids do recover well from the virus. But not always.

Measles symptoms tend to start with a fever and a runny nose. The telltale rash comes later. In some cases, severe complications develop. They can include pneumonia, blindness, and inflammation of the brain. Some people won’t develop complications until years later. In rare cases, the disease can be fatal.

Before the measles vaccine was introduced, in 1963, measles epidemics occurred every two to three years, according to the WHO. Back then, around 2.6 million people died from measles every year. Since it was introduced, the measles vaccine is thought to have prevented almost 59 million deaths.

But vaccination rates have been lagging, says Anne Zink, an emergency medicine physician and clinical fellow at the Yale School of Public Health. “We’ve seen a slow decline in people who are willing to get vaccinated against measles for some time,” she says. “As we get more and more people who are at risk because they’re unvaccinated, the higher the chances that the disease can then spread and take off.”