Washington has declared a state of emergency over a measles outbreak that's occurred primarily among unvaccinated children in a southwestern county.

In Clark County , health officials have identified 35 confirmed cases of measles since Jan. 1. At least 31 patients had not been vaccinated against the highly contagious viral infection. The immunization status for four cases was not verified, and another 11 cases are suspected.

Twenty-five of the confirmed cases were among children 10 or younger, while just one case was among a patient older than 18. One child was hospitalized, and a man from King County, home to Seattle, who had visited Clark County also contracted measles , local health officials said .

"The measles outbreak and its effects impact the life and health of our people, as well as the economy of Washington State, and is a public disaster that affects life, health, property or the public peace," Gov. Jay Inslee said in a proclamation declaring the statewide emergency.

Clark County – home to about 475,000 people and situated outside Portland, Oregon – declared a local emergency Jan. 18. Low vaccination rates make the Portland area a "hot spot" for outbreaks, Peter Hotez, a professor and dean at the Baylor College of Medicine in Houston, told The Seattle Times.

State data show only about 77 percent of Clark County kindergarteners had completed their vaccinations for the 2017-2018 school year, down from 91.4 percent in 2004-2005. Health experts say roughly 95 percent of people should be vaccinated to create "herd immunity" against a contagious disease like measles.

"This is something I've predicted for a while now," Hotez told The Seattle Times . "It's really awful and really tragic and totally preventable."

The spread of measles in Clark County "creates an extreme public health risk that may quickly spread to other counties," the governor's proclamation said .

The virus is so contagious that nearly everyone who comes into contact with it and isn't immune will become infected, according to the Centers for Disease Control and Prevention. In rare cases, measles can be deadly.

The virus can linger in the air for up to two hours after an infected person leaves the area. Measles can be spread through coughing or sneezing and cause fever, cough, runny nose, red eyes and a rash.