A measles outbreak in British Columbia has crossed over the border into the Pacific Northwest, health officials in Canada and United States have confirmed.

Health officers in B.C.'s Fraser Valley, southeast of Vancouver, say they received more than 350 reports of the childhood disease since an outbreak began there in early March.

Now, six cases of measles have been diagnosed in Washington state's Whatcom County, according to a report by National Public Radio affiliate station KPLU.

One of the cases south of the border has prompted a region-wide alert, since it's believed the infected 20-something woman may have mingled with crowds at a rock concert in Seattle while she was contagious.

She also apparently visited tourist attractions throughout the Puget Sound area, including Seattle's famed Pike Place Market, LeMay Car Museum and Harmon Brewing Company in Tacoma.

Whatcom County Health Officer Greg Stern says this measles outbreak traces back to a community in British Columbia in which families typically avoid vaccinating their children.

"To the extent that people avoid vaccines, they increase both their risk and the risk of the community so that it can take hold. I'm worried about that," Stern said in the KPLU report.

Measles, in fact, say health experts, is easily prevented with a vaccine.

According to the U.S. Centers for Disease Control and Prevention, "measles causes fever, runny nose, cough and a rash all over the body. About one out of 10 children with measles also gets an ear infection, and up to one out of 20 gets pneumonia. For every 1,000 children who get measles, one or two will die."

Seven cases of measles have so far this year been reported to the state Department of Health, compared to just five throughout all of 2013.

The Washington and Oregon legislatures have in recent sessions made it harder to get vaccination exemptions for school-age children.