By LISA BAUMANN ,  Associated Press

SEATTLE (AP) — Health officials have confirmed 26 measles cases in western Washington in an outbreak that began earlier this month.

In Clark County, where the outbreak has been centered, the number of cases rose to 25 as of Thursday, with a dozen more suspected cases under investigation, said Dr. Scott Lindquist, epidemiologist for communicable diseases with the Department of Health.

King County also confirmed one case this week involving a man in his 50s who had recently visited Clark County, he said.

"We usually have five cases or less a year," he said. "At this point, we're expecting it to move across the state."

Twenty-one of the people infected were not immunized, according to officials. The others are not verified to have had the vaccine.

Nineteen of the Clark County cases involve children younger than 10. Five cases involve those from 11 to 18, and one infected person is between 19 and 29 years old.

Measles is a highly contagious viral illness spread when an infected person coughs or sneezes. It can remain in the air for up to two hours in an isolated space.

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, Lindquist said. "You feel miserable," he said.

While many people feel like measles are not a big deal, officials are concerned because serious complications such pneumonia and brain infections can arise, Lindquist said. He also urged people who have not received the vaccine to get it now.

Those who are infected have visited public places while contagious, including the Portland International Airport, health care facilities and schools.

The Clark County Public Health said Thursday the people newly confirmed to have measles have stayed home, preventing additional exposures to the public. County officials on Friday declared a public health emergency over the outbreak.

People who think they may have the measles are asked to contact their health care provider before visiting to avoid exposing others.