As of Sunday afternoon, there were 34 confirmed cases of measles in Clark County, just north of Portland, Oregon, and one in King County, which includes Seattle.
"I've been saying now for the last couple of years [that] it's only a matter of time before we see a horrific measles outbreak in the Pacific Northwest", said Dr. Peter Hotez, co-director of the Texas Children's Hospital Center for Vaccine Development.
And at least 30 of the infections are people who haven't had the measles vaccination, majority children between the ages of one and ten.
There has also been an adult case.
Clark County - which sits across the Columbia River from Portland, Oregon - has led Oregon officials to warn residents about potential areas of exposure.
Here's why the current outbreak in Washington is a flawless 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.
Inslee's proclamation allows agencies and departments to use state resources and "do everything reasonably possible to assist affected areas".
Measles was a deadly disease for children in the USA before the vaccine became available in 1963.
An infected person was at the Portland International Airport on January 7 - at Concourse D and the Delta Sky Lounge - and at baggage claim and the south end of the ticket counter on January 15, reported The Columbian.
Also, because of how the virus is spread through sneezing and coughing, the outbreak in Washington is concerning even though it's happening on the other side of the country.
"The message that we give to people is, please immunize your children".
These states have particularly high levels - or "hot spots" - of unvaccinated children, and are vulnerable to measles outbreaks, according to a study by Hotez and his colleagues that published past year.
Measles is a highly infectious viral respiratory disease that spreads through the air and through direct contact with respiratory body fluids like snot and saliva.
The most recognizable symptom is a red, blotchy rash.
Doctors say the full vaccine is 97 per cent effective and protects you for life.
Washington state has one of the lowest vaccination rates when it comes to measles. Measles was declared eliminated - defined by absence of continuous disease transmission for greater than 12 months - from the United States in 2000. This is much higher than the national average of children unvaccinated for nonmedical reasons, which is estimated at 2 percent nationally.
To keep your child safe and strengthen herd immunity, be assured that there is absolutely no evidence to support the idea that vaccines cause autism. But it raises the possibility the Washington outbreak could spread across the state line. Measles was also a leading cause of death for children globally. OR is close to Clark County and this has sparked worries. So if a person with measles was at your house, it is pretty likely that if you're not vaccinated, that you will most likely contract the ailment.