VANCOUVER, Wash. — The number of confirmed measles cases near Portland grew to 31 on Friday — an outbreak boosted by lower-than-normal vaccination rates in what has been called an anti-vaccination U.S. "hot spot."

Public health officials in southwest Washington, just across the Columbia River from Portland, Oregon, said people may have been exposed to the dangerous disease at more than three dozen locations , including Portland International Airport, a Portland Trail Blazers game, an Amazon Locker location and stores such as Costco and Ikea.

Twenty-six of the confirmed patients had not been vaccinated against measles, and the vaccination status of four others who were infected is unknown. One child has been hospitalized. Authorities say nine additional cases are suspected.

One case also has been confirmed in King County, which is home to Seattle and one was confirmed Friday evening in Multnomah County, which is home to Portland.

Most of the cases involved children younger than 10, the Clark County Public Health Department said in a statement. One adult is infected, and the rest are teenagers. Oregon officials didn't provide the age of the adult infected there.

A sign prohibiting all children under 12 and unvaccinated adults stands at the entrance to PeaceHealth Southwest Medical Center in Vancouver, Wash., Friday, Jan. 25, 2019. Washington Gov. Jay Inslee declared a statewide public health emergency Friday as confirmed measles cases rose to 30, with nine more suspected. AP Photo/Gillian Flaccus

Washington Gov. Jay Inslee, a Democrat, declared a statewide public health emergency for his state on Friday. Authorities in neighboring Oregon and Idaho have issued warnings.

Inslee said the number of cases "creates an extreme public health risk that may quickly spread to other counties."

The measles vaccine has been part of routine childhood shots for decades, and measles was declared eliminated in the U.S. in 2000.

But measles is still a big problem in other parts of the world. Travelers infected abroad can bring the virus into the country and spread it, causing periodic outbreaks.

Last year, there were 17 outbreaks and about 350 cases of measles in the U.S.

Officials still are not sure where this Pacific Northwest outbreak began. The first known patient sought medical care on Dec. 31, but it is unknown if other people may have gotten sick before that and did not seek treatment. Public health officials are focused for now on preventing more exposures.

It could be weeks or even months before the "exquisitely contagious" virus runs its course in Washington, Dr. Alan Melnick, the Clark County health officer, said Friday.

People who choose not to vaccinate their children are underestimating the dangers of the illness, said Melnick, who himself had measles as a child, before the vaccine was commonplace.

Before the vaccine, 400 to 500 people died from the measles each year, 50,000 people were hospitalized and 4,000 people developed brain swelling that can cause deafness, he said. Between one and three cases out of every 1,000 are fatal, he said.

"It's one of the most contagious viruses we have. It can have really serious complications ... and it's entirely preventable with an incredibly cheap and safe vaccine," Melnick said.

Clark County has already spent more than $100,000 trying to contain the outbreak, and staff is being pulled from other duties, including restaurant inspections, he said.

"It's all hands on deck. Clearly this is going to cost hundreds of thousands of dollars, and it wouldn't surprise me if we were in the seven figures by the time we're done here," he said. "These costs could have been prevented if we had everybody vaccinated."

Clark County, which includes the Portland bedroom community of Vancouver, Washington, has a measles vaccination rate of 78 percent, well below the 92 to 94 percent rate required for so-called "herd immunity," said Marissa Armstrong, the department's spokeswoman. Herd immunity happens when unvaccinated individuals are protected from infection because almost everyone around them has been vaccinated and is immune to a disease.

The measles vaccination rate for 2-year-olds in Multnomah County, home to Portland, was 87 percent in 2017, according to state data. The measles vaccine consists of two shots, one given by age 2 and the second usually between ages 4 and 6.

Data on Portland's vaccination rate for both shots wasn't immediately available.

Two doses of the vaccine in childhood are 97 percent effective and provide lifetime immunity. One dose is about 93 percent effective.

Both Washington and Oregon allow vaccine exemptions for personal and philosophical reasons. The vaccine-exemption rate in Clark County for non-medical reasons was high, at 7.5 percent, Armstrong said.

The incubation period for measles is seven to 21 days, which means that an unvaccinated person who has been exposed could be out in public for up to three weeks before getting sick. Patients remain contagious for four days after they develop the rash.

The virus, spread by coughing or sneezing, can remain in the air for up to two hours in an isolated space. Ninety percent of people exposed to measles who have not been vaccinated will get it, public health officials said.

Every time an unvaccinated person who has been exposed to measles goes out in public, "it starts that clock over again," Armstrong said.

Earlier this week, authorities were successful in identifying several people who had been exposed but were not sick yet. Those people stayed home and later got ill, Armstrong said.

Those who may have been exposed should watch for early symptoms of fever and malaise and then a rash starting on the head and moving down the body. Serious complications such pneumonia and brain infections can arise from the disease in some cases.