A growing measles outbreak in Washington state led the governor to declare a state of emergency on Friday. Now, 35 cases have been confirmed in Clark County, a southwestern county that includes Vancouver, and 11 potential cases are awaiting confirmation. There has also been one confirmed case in King County, which includes Seattle.

So why the emergency declaration? Because measles is a highly contagious disease. The virus can live for up to two hours wherever an infected person has coughed or sneezed. According to The Centers for Disease Control and Prevention, 90 percent of non-vaccinated individuals exposed to someone with the disease will also become infected.

Once upon a time, the United States had eliminated the measles, but outbreaks have increased as the anti-vaccine movement takes hold — and 18 states now allow for non-medical “philosophical” vaccination exemptions.

Here’s why the current outbreak in Washington is a perfect example of what can happen when parents do not vaccinate their children for philosophical or religious reasons — and how exactly that can put their own kids, and others, at risk.

Most of the cases have been in kids. And most of those kids are un-immunized.

Twenty-five of the 35 people in Clark County who’ve caught the measles are kids between the ages of 1 and 10, while another nine are between 11 and 18. So far, just one adult in the area has caught the disease.

Thirty-one of those 35 people were unvaccinated. The vaccination status for the remaining four is, for now, unclear.

What is clear to experts is that not being vaccinated is what put those people at risk — and what allowed the measles to spread. Public health experts recommend children get two doses of the MMR vaccine: the first when they’re 12 months old, and a second between the ages of 4 and 6.

In Clark County, however, vaccination rates have been dropping. In the 2004-2005 school year, 96 percent of Clark County kindergartners had been vaccinated against the measles, KATU 2 reports. By 2017-2018, only 84 percent of kindergartners had.

“The fact is, this never would have happened if all of those children were immunized, or even if most of them were immunized,” John Lynch, medical director of infection control at UW Medicine’s Harborview Medical Center, told HuffPost. “Being vaccinated provides very good protection against the measles.”

“This never would have happened if all of those children were immunized, or even if most of them were immunized.” - John Lynch, Harborview Medical Center

Indeed, recent measles outbreaks like the one in Disneyland in 2015 are thought to be fueled by vaccine refusal as vaccination rates drop below the levels needed to provide herd immunity.

“As a pediatrician and parent, I find it frustrating because this is happening among children who are dependent on their parents’ decision-making,” said Wendy Swanson, a pediatrician and chief of digital innovation at Seattle Children’s Hospital.

“We know that 31 of the cases so far are in un-immunized people, and these are people who are in the age-range that immunization is recommended. And we have a vaccine that is known to be very safe and wildly effective.”

A lot of kids who can’t be vaccinated are now at risk.

Babies typically aren’t given their first dose of the measles vaccine until their first birthday (though there are exceptions for babies who travel out of the country), in part because mothers who’ve been immunized against the disease either through vaccination or because they once had the disease pass some of that immunity on to their little ones.

“If you put a vaccine in while maternal antibodies are still around, the vaccine won’t stimulate the baby’s own immune system to respond, it will just get soaked up by the maternal antibodies doing their job,” Swanson explained in a 2015 blog post on the topic.

Thus far in Washington, no cases have been in children under the age of 1, which Swanson believes is at least partly because local officials have been dogged in their efforts to track exactly where infected individuals have been (schools, daycares, health centers, grocery stores, etc.) so parents with babies can stay away.

AP Photo/Gillian Flaccus, File In this Jan. 25, 2019 photo, a sign at the entrance to PeaceHealth Southwest Medical Center in Vancouver, Washington prohibits all children under 12 and unvaccinated adults due to the number of measles cases in Clark County.

But children who are immunocompromised — like those who are on steroids or who are undergoing chemotherapy, for example — are also unable to be vaccinated.

“It’s the kids who are too young to be immunized, or who are too sick to be immunized that we worry about,” Swanson said. “They’re not going to be protected.”

An outbreak like the one in Washington shows how easily the disease can spread among unvaccinated children and that once there is a pocket of the measles, it can easily spread. The Oregonian reports that two Clark County children who contracted the disease and who were unvaccinated recently traveled to Hawaii, where they were quarantined and then flew home after five days.

The good news? Most parents are onboard with vaccinating their kids.

“If your kids are immunized, you should feel really comfortable in the face of an outbreak,” Swanson said. Two doses of the measles vaccine are about 97 percent effective at preventing the disease, the CDC says, but even just one dose (which is what most kids under age 4 have) is 93 percent effective.

Swanson worries that sometimes the media coverage about measles outbreaks makes it seem there is a nationwide crisis over vaccination, but that’s not true. More than 90 percent of kids in this country are vaccinated, according to public health recommendations.