Authorities did not identify the woman, but said she was likely exposed to measles at a local health facility during a recent outbreak in Clallam County.
She died in the spring. The measles infection was discovered during an autopsy.
"The woman had several other health conditions and was on medications that contributed to a suppressed immune system. She didn't have some of the common symptoms of measles such as a rash, so the infection wasn't discovered until after her death. The cause of death was pneumonia due to measles," the health department said.
It stressed that no one who had contact with any of the known cases remains at risk.
So far this year, 11 people have been diagnosed with measles in Washington state, including six in Clallam County, according to the health department.
"The last active case of measles in Washington this year was reported in late April. Within about three weeks of exposure to someone with measles, it's possible to develop the disease. Since more than three weeks has already passed since the last active measles case, no one who had contact with one of the known cases is any longer at risk for developing measles from those exposures," it said.
Measles is a highly contagious respiratory disease. It causes fever, red and sore eyes, runny nose, cough and a rash. It can cause deadly health complications, including pneumonia and encephalitis. It is spread by contact with an infected person through coughing or sneezing. It can remain in the air and on surfaces for up to two hours.
While once widespread in the United States, cases dropped significantly because of vaccines
. In 2000, health authorities declared that measles had been eliminated in the United States, which meant it was no longer native but continued to be brought in by international travelers.
In 2008, 2011, 2013 and 2014, there were more reported measles cases compared with previous years, according to the Centers for Disease Control and Prevention.
The CDC attributes this to two things: more measles cases coming into the United States, and more spreading of the disease in communities with pockets of people who are not vaccinated.
Just this week, California Gov. Jerry Brown signed legislation
outlawing a family's personal and religious beliefs as reasons to exempt their children from school vaccinations.
The controversial proposal, arising partly from concerns over this year's Disneyland outbreak, would allow medical exemptions deemed appropriate by the state Department of Public Health.
From January until May 29, 173 people in 21 states and the District of Columbia developed measles, and 117 of those cases were linked to Disneyland in Orange County, California, according to the CDC.