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.

Heres 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 whove 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 theyre 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 Medicines Harborview Medical Center, told HuffPost. Being vaccinated provides very good protection against the measles.

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 Childrens 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 cant be vaccinated are now at risk.

Babies typically arent 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 whove 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 wont stimulate the babys 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.