VANCOUVER, Washington—The number of confirmed measles cases near Portland grew to 31 on Jan. 25.

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.

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.

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.

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.

By Gillian Flaccus