Last week, Washington Gov. Jay Inslee declared a public health emergency after confirming more than two dozen cases of measles. By Tuesday, the number rose to 37 — most of them affecting children.

The majority of the patients — 36 out of 37 — popped up in Clark County, which sits along the state border next to Portland, Oregon. Of this group, 35 were children under 18, and at least 32 hadn’t been vaccinated for measles, mumps and rubella.

“Measles is exquisitely contagious,” said Alan Melnick, Clark County’s public health director. Melnick said that if 10 people without immunity walked into a room with someone with measles, nine would become infected. Since it can be passed through coughing or sneezing, the room could stay infectious for up to two hours after the person with measles leaves.

If 10 people without immunity walked into a room with someone with measles, nine would become infected.

Portland identified its first case of measles on Friday, the state health department confirmed to the PBS NewsHour. The Clark County’s health department reported that patients have traveled around the two states, from a professional basketball game to churches and schools, potentially infecting others.

As NewsHour reported in October, Oregon is especially vulnerable to preventable infectious disease outbreaks because its schoolchildren have the highest exemption rates for vaccines in the country.

Paul Cieslak, a medical director in the communicable disease prevention and immunization department at Oregon Health Authority, said that this is already the biggest outbreak in the Portland area since 1996.

Here’s a look into how this epidemic started, why children may face the worst of it, and how it is no surprise that the outbreak hit this part of the country:

Where did this measles outbreak come from?

“One of the reasons that we need to keep our vaccination rates up in the United States is because measles still exists in the world, and it’s only a plane ride away,” Melnick said.

The Centers for Disease Control and Prevention declared in 2000 that measles was eliminated from the U.S., but travelers exposed to outbreaks abroad can carry the disease to the states, potentially wreaking havoc among small pockets of unvaccinated people. Scott Lindquist, Washington state’s epidemiologist for communicable diseases, thinks this scenario is most likely what prompted the current outbreak in Washington.

The World Health Organization saw a dramatic increase in infections and extended outbreaks last year in Europe. More than 41,000 people were infected within the first six months of 2018, surpassing the 12-month totals for every other year this decade. If an infected traveler comes from a country with persistent measles outbreaks, such as Ukraine or Israel, they could unwittingly bring the disease into the U.S. and infect people domestically.

“We have a large unvaccinated population in Clark County,” Melnick said. “We don’t have sufficient herd immunity here to keep this from spreading.”

Why this outbreak puts children at risk

Melnick said that the outbreak is mostly affecting unvaccinated children for a few reasons. First, medical experts consider anyone born before 1957 to have natural immunity, since there’s such a high chance that they had measles as a child.

Additionally, children tend to congregate together in close spaces, such as schools.

Melnick pointed out that it’s “fairly easy” to get a philosophical or personal exemption for school vaccination requirements in Washington state.

In one Clark County school district, less than 44 percent of kindergarteners have completed their required immunizations for the 2017-2018 school year. Melnick thinks that misinformation about measles and vaccines on social media is a major contributor to the problem.

If you suspect that you or someone you know may have measles, don’t run to the doctor’s office.

“All you have to do is bring measles into a community where you have a large unvaccinated population, and it can spread like wildfire,” Melnick said.

This outbreak could be a big threat for unvaccinated children in Oregon. Child vaccine exemption rates for the state jumped from 1 percent in 2000 to 7.5 percent by the 2016-2017 school year.

These pockets also exist nationally. The CDC reported in October that despite an overall increase in vaccination rates since 2001, unvaccinated communities have grown for the third consecutive year.

Measles can affect others too. While the disease isn’t a threat to those with immunity, it spells danger for the unvaccinated, pregnant, very young and immunocompromised. Measles can have serious and potentially deadly complications, including pneumonia and encephalitis, a brain infection.

Pregnant women without immunity risk spontaneous abortions from the disease, and infants can’t receive their first vaccination until they reach 12 months. In 2015, a woman in Washington with a compromised immune system became the first patient to die of measles in 12 years after she was exposed.

How health officials are taking action

Melnick also pointed out a few ways that vaccines can’t impact you: Vaccines don’t contain harmful amounts of aluminum or mercury — a common notion propagated by the anti-vaccine community — and can’t give you autism, which has been debunked so many times that each letter of that phrase is linked to a different source. Febrile seizures, which have often been mistakenly linked to brain damage and autism, are actually more common with measles-induced fevers than from the vaccine.

By the way, the measles vaccine is extremely effective. One dose provides 93 percent immunity, and the second raises immunity levels to 97 percent.

For now, officials in Washington and Oregon are primarily focused on identifying and monitoring infected patients. Children diagnosed with measles are being kept out of school to prevent further spread. All of this takes up a lot of government resources, and it could continue for weeks.

Someone infected with measles won’t show symptoms for about two weeks. Due to this incubation period, a measles outbreak will typically have multiple “generations” of cases, Cieslak said.

If not properly controlled, Melnick said that this outbreak, which was identified on Dec. 31, 2018, could last for weeks or months.

If you suspect that you or someone you know may have measles, all the experts the NewsHour spoke to said the same thing: Don’t run to the doctor’s office. By bringing measles to a medical center, you could pass along the disease to unsuspecting patients. Instead, call the office beforehand to receive instructions.

The outbreak has also prompted Washington lawmakers to propose new legislation, which would ban philosophical and personal exemptions for the MMR vaccine for children enrolled in private and public schools, as well as daycare centers.