A sixth case of measles has been confirmed in Western Washington this week. The most recent case was diagnosed in a King County woman in her 40s. 

She is the third known case in King County. Two other cases were confirmed in Pierce County and one in Snohomish County. 

On Friday, health officials announced the woman visited several locations in King County before knowing she was contagious. 

In all six cases, the common exposure site is Sea-Tac Airport on April 25, most likely in the morning. 

According to the Public Health Department of Seattle & King County, "Health officials have no reason to believe that there is currently an increased risk of getting measles by visiting the airport." 

RELATED: List of possible measles exposure sites in Western Washington

On Thursday, classes at Issaquah High School were cancelled after an employee at the school was diagnosed with the measles virus. The district conducted a vaccination check of staff members and school resumed on Friday. 

RELATED: Do teachers in Washington need to be vaccinated for measles?

Another school is also involved in the May measles outbreak. A Snohomish County student diagnosed with measles attends North Creek High School in Bothell. 

The Northshore School District reports that nearly 99 percent of North Creek students have the measles, mumps, and rubella vaccine (known as MMR). There are 10 students without proof of immunization who will be excluded from school until June 3, barring any additional confirmed cases. 

RELATED: Most Washington schools are failing to meet state's target for vaccinations

Measles is highly contagious and causes fever, rash, cough, and watery eyes. Symptoms can begin seven to 21 days after exposure, and is contagious from about four days before the rash appears to four days after it ends.

The disease can be prevented with the MMR vaccine. In two doses, the vaccine is over 95 percent effective at preventing measles.

RELATED: Washington state limits exemptions for measles vaccine