A measles outbreak in Washington State has prompted health officials to declare a public emergency. According to NPR’s Vanessa Romo and Patti Neighmond, there were 36 confirmed and 11 suspected measles cases as of Monday.

Thirty-five of the confirmed measles patients and all of the suspected cases are located in Clark County. All but four of these patients had not been immunized with the measles-mumps-rubella vaccine; officials are working to verify the immunization status of the remaining individuals. Twenty-five of the 35 confirmed patients are children under 10 years old. One adult case has also been identified in nearby Kings County, according to Vox’s Julia Belluz.

Clark County officials have compiled a list of dozens of public places—among them stores, medical clinics, churches, schools and the Portland International Airport—where people may have been exposed to the infection. In a statement declaring the state of emergency, Washington Governor Jay Inslee called the situation an “extreme public health risk that may quickly spread to other counties.”

Measles is a highly contagious viral illness. According to the CDC, 90 percent of non-immunized individuals near an infected person will contract it. Measles spreads through the air, and the virus can live for up to two hours in the airspace where an infected person coughed or sneezed. It can also be difficult to catch early signs of the illness; symptoms generally start to appear seven to 14 days after infection, meaning that people can spread the disease before they know they are sick.

Telltale measles symptoms include high fevers, red and watery eyes, white spots in the mouth and skin rashes. The illness can have serious complications, such as pneumonia and encephalitis, or swelling of the brain. Some complications prove fatal.

Prior to 1963, when the measles vaccine became available, measles was the leading cause of death among children worldwide, report Romo and Neighmond. According to the CDC, it infected between three and four million people in the United States every year, causing an estimated 400 to 500 deaths. The introduction of the vaccine, which is 97 percent effective after two doses, dramatically reduced measles rates in the country. In 2000, officials declared that measles had been officially eliminated from the U.S.

But in recent years, outbreaks have been cropping up in pockets of the country where the anti-vaccination movement has taken hold, fuelled by disproven claims that vaccines cause autism. Eighteen American states, including Washington and Oregon, allow parents to opt out of vaccinating their children due to “philosophical beliefs,” reports CBS News. Several metropolitan areas in the Northwest—Seattle, Spokane and Portland—have been identified as “hotspots” of vaccine opposition.

In Clark County, which borders on Portland, nearly seven percent of children were exempted from required kindergarten entry vaccines in the 2017-2018 school year for non-medical reasons, according to Isaac Stanley-Becker of the Washington Post. Nationwide, only two percent of children are not immunized due to non-medical concerns.

If a high percentage of a given population are immunized against a disease, vaccines can protect event those who are not immune. But given the high rate of non-vaccination in “hotspot” areas, Peter J. Hotez, a professor of pediatrics and dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, tells Stanley-Becker that he isn’t surprised that outbreaks are happening.

“This is something I’ve predicted for a while now,” he says of the situation in Clark County. “It’s really awful and really tragic and totally preventable.”