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.

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.