Washington Department of Health officials have declared a state of emergency as they scramble to contain a measles outbreak in two counties in the state. There were 36 confirmed cases and 11 suspected cases of the potentially deadly virus as the number continues to rise in a region of the United States with a lower-than-normal vaccination rate.

In Clark County, Washington, which borders Portland, Oregon, Monday’s 36 confirmed measles cases were up significantly from the 26 on Friday, when Governor Jay Inslee declared a state of emergency. Most of the measles cases involved children between 1 and 10 years old who had not been vaccinated. Health officials anticipate that the outbreak will rapidly expand.

In a statement Friday, Inslee said: “The measles virus is a highly contagious infectious disease that can be fatal in small children” and that the number of confirmed cases “creates an extreme public health risk that may quickly spread to other counties.”

The outbreak of measles in Washington state is taking place nearly two decades after the virus was eliminated in 2000. By that time, enough people were immunized that outbreaks were uncommon and deaths from measles were virtually unheard of. A rise in the percentage of unvaccinated children has directly led to a rise in the number of cases of the potentially fatal disease.

Washington and Oregon are some of the more permissive states in the US in allowing parents to opt out of vaccines, including for measles. In Clark County, 7.9 percent of children were given exemptions from vaccines for entry to kindergarten in the 2017-18 school year, according to the Washington Post. This is much higher than the national average of children unvaccinated for nonmedical reasons, which is estimated at 2 percent nationally.

Before the measles vaccine was introduced in the US in 1963, there were 4 million measles cases with 48,000 hospitalizations and 500 deaths in the US every year. Measles was also a leading cause of death for children globally. Intense pockets of transmission still exist around the world today, especially in low-income countries such as the Philippines and Vietnam.

In addition to the US, there has also been an uptick of people contracting measles in Canada and across Europe due to people forgoing the vaccine. Europe saw more than 41,000 measles cases in the first half of 2018, a record high in the post-vaccine era. The tragedy is that the deadly disease and its consequences are almost 100 percent preventable if the population is vaccinated.

Measles strikes after an incubation period of 10 to 12 days in the form of fever, cough, stuffy nose, and bloodshot and watery eyes. Sufferers, overwhelmingly children, can be hit with loss of appetite, malaise and confusion. Several days after these initial symptoms, an uncomfortable rash spreads from the face and neck downward through the rest of the body.

In uncomplicated cases, sufferers usually begin to recover as soon as the rash appears and feel better in about two to three weeks. However, up to 40 percent of patients have complications, usually occurring in children under five, adults over 20, and anyone who is undernourished or immunocompromised.

Children under age 5 have the highest probability of death from measles. Pneumonia is the most common complication, accounting for most measles-related deaths. Less common complications include blindness, croup, mouth ulcers, ear infections, and severe diarrhea. Some children develop swelling of the brain, or encephalitis, which can lead to convulsions, loss of hearing and mental retardation.

A measles outbreak in the US usually begins when a traveler picks up the virus in another country where measles is still common. If that person brings the virus back to a community with a high rate of unvaccinated individuals, it can spread rapidly.

The measles virus is airborne, and is transmitted by respiratory droplets from the nose, mouth, or throat of an infected person, usually through coughing or sneezing. Small-particle aerosols from an infected person can remain suspended in air for long periods of time after a person has left a location, and the virus can live on surfaces for up to two hours. The virus can spread in a person four days before the onset of the rash associated with measles, so people carrying the virus can spread it to others before even knowing they have the disease.

In Washington state, people with the measles virus reportedly visited public places such as healthcare facilities, schools and churches, as well as stores such as Dollar Tree and Ikea, potentially spreading the disease.

The measles vaccine is given as part of the combination MMR (measles, mumps, rubella) injection. The US Centers for Disease Control and Prevention (CDC) recommends that children receive two doses: one at 12 through 15 months of age, the second at 4 through 6 years of age.

As the MMR vaccine cannot be administered until age 1 this leaves these young infants at particular risk of contracting the measles if they come into contact with the virus.

According to the CDC, fevers after the MMR vaccine occur in one in six people, mild rashes in one in 20. More severe problems are virtually nonexistent, with serious allergic reactions happen in fewer than one in a million cases.

Overall, MMR vaccine refusal by parents is not that common, with about 91 percent of young children receiving the shot in 2016, according to the CDC. This is nearly enough for “herd immunity,” in which a certain percentage of the population needs to be immunized.

“Herd immunity” means that a disease cannot spread very easily, even among those who can’t be vaccinated, like newborns and those with vaccine allergies. However, despite the national rate of vaccination, there are geographical clusters of unvaccinated people. These include vaccine-averse Amish in Ohio, Orthodox Jews in New York, as well as parents who choose not to put “unnatural” substances in their bodies or who choose to delay immunizing their children.

According to a 2018 analysis published in PLOS Medicine, dozens of counties across the country had nonmedical exemption rates exceeding the national average. In 2016-17, Camas County, Idaho, led the nation with a 27 percent opt-out rate.

A movement of parents in the US claims that vaccines, including MMR, cause autism, Attention-Deficit/Hyperactivity Disorder (ADHD) and other developmental problems in children. These theories have been promoted by figures such as Robert F. Kennedy, Jr., who met with then president-elect Donald Trump during the transition period in January 2017 to discuss plans to chair a commission “on vaccine safety and scientific integrity.”

During the 2016 presidential campaign, Trump spoke with a group of donors at a Florida fundraiser who are prominent proponents of the discredited link between vaccines and autism, including disbarred British physician Andrew Wakefield. Wakefield was the senior author of a now retracted 1998 Lancet study linking autism to the MMR vaccine. The study involved only 12 children, including 8 whose parents were already convinced of the MMR-autism connection.

Trump in 2016, playing to the anti-vaccine community, stated without substantiation: “You take this little beautiful baby and you pump—I mean, it looks like just it’s meant for a horse and not a child…We had so many instances, people that work for me, just the other day, 2 years old a beautiful child, went to have the vaccine and came back and a week later got a tremendous fever, got very, very sick, now is autistic.”

It seems the Trump administration’s plans to launch an anti-vaccine commission have for the time being been stalled, much to the dismay of Kennedy and the other vaccine opponents. However, the fractured state of the public health system in the US allows millions of parents to be granted exemptions by states and localities from their children receiving the MMR vaccine.

The current state of emergency in Washington state points to the danger—on the basis of unsubstantiated and antiscientific theories—of measles and other diseases, long thought to be eradicated in North America and Europe, to reemerge.