Yet another measles outbreak is endangering children — this one in Washington state. Of the 37 people already infected in this outbreak, at least 32 weren’t vaccinated against measles, and 35 were under 18 years old. Experts say this outbreak was easy enough to predict, and could have been prevented if more children in the area were vaccinated. The situation raises the question — how many more kids will get sick before vaccination rates increase?

Measles has been flaring up around the world, with outbreaks in the US cropping up in New York, New Jersey, and now, Washington state. Last year, 349 people were infected with measles — making it the second-worst measles year since 2000, when health officials determined that the disease had been eliminated in the US. This deadly infection can be prevented with the safe and effective measles vaccine, but not every state has closed the loopholes that allow parents to exempt their kids from vaccination for non-medical reasons.

“They have to live in fear of walking out with their infant into Walmart, or the public library.”

In Clark County, Washington, where at least 25 children younger than 10 years old are sick with measles, nearly 8 percent of children entering kindergarten in 2017 had skipped their required vaccines, according to The Washington Post. That’s worrying because the virus is particularly dangerous to kids. It’s well known for causing a rash and fever, but it can also cause pneumonia, brain damage, and death. And it’s incredibly contagious, according to the CDC. Being in the same room as someone who has measles — even two hours after they’ve left it — can put you at risk if you’re not vaccinated.

That’s why the measles, mumps, and rubella (MMR) vaccine is so important; people who are too young or too sick to get the vaccine rely on everyone around them getting vaccinated to stay healthy. “Once a measles outbreak starts, the ones who are the most victimized are the infants under the age of 12 months not yet old enough to get their measles vaccines,” says Peter Hotez, dean for the National School of Tropical Medicine at Baylor College of Medicine. “A parent in Clark County, Washington — they have to live in fear of walking out with their infant into Walmart, or the public library.”

“It’s so frustrating because you can see this coming down the tracks.”

All 50 states in the US require that kids receive certain vaccinations before starting school, but there are loopholes that allow kids to go unvaccinated. There are medical exemptions for kids who can’t get vaccines for health reasons, and religious exemptions for kids whose religions forbid it. There are also 17 states that allow parents to skip their kids’ vaccinations because of their personal beliefs, according to the National Conference of State Legislatures. Washington state is one of them, for now. As the number of measles cases climbs, a newly introduced bill could change that — at least, for the MMR vaccine, according to King 5 News.

We’ve seen state legislatures change their tune and tighten those loopholes after massive outbreaks in the past. In late 2014 and early 2015, 147 people were infected with measles in an outbreak linked to Disneyland in California. Most of them were unvaccinated, according to the CDC. In response, California passed a law that rolled back non-medical vaccination exemptions — making it one of only three states to do so, along with Mississippi and West Virginia.

“In public health, we never want more outbreaks.”

It’s a case study for the kind of change that can come after a major outbreak, says Leila Barraza, an assistant professor at the University of Arizona’s College of Public Health. “In public health, we never want more outbreaks,” she says. But outbreaks can prompt efforts to change policy, Barraza wrote for the Journal of the American Medical Association back in 2015. After the Disneyland outbreaks, lawmakers across the country worked to change vaccine laws, and a few were successful — like California, and Vermont, which cut the state’s philosophical vaccine exemption. “Anytime you have a big outbreak like this it really does bring vaccination and vaccine-preventable diseases to the forefront of everyone’s minds,” she says.

But outbreaks are a steep price to pay for change, particularly because the consequences of vaccine hesitancy are so obvious, Hotez says. “It’s so frustrating because you can see this coming down the tracks.”