The woman lacked some of the measles' common symptoms, such as a rash, so the infection was not discovered until an autopsy, Washington State Department of Health spokesman Donn Moyer said.
This is the 11th case of measles in Washington — and the sixth in Clallam County — this year, Moyer said.
Measles is highly contagious and spreads when an infected person breathes, coughs or sneezes. However, dying from it is extremely rare, Moyer said.
Officials didn't say whether the woman was vaccinated, but they did note she had a compromised immune system. They withheld her age to protect her identity but said she was not elderly.
The woman was hospitalized for several health conditions in the spring at a facility in Clallam County, which covers the northern part of the Olympic Peninsula. She was there at the same time as a person who later developed a rash and was contagious for measles, Moyer said. That's when the woman most likely was exposed.
She was on medications that contributed to her weakened immune system, he said.
After being treated in Clallam County, the woman was moved to the University of Washington Medical Center in Seattle, where she died. An autopsy concluded the cause of death was pneumonia due to measles.
"This tragic situation illustrates the importance of immunizing as many people as possible to provide a high level of community protection against measles," Moyer said. "People with compromised immune systems cannot be vaccinated against measles. Even when vaccinated, they may not have a good immune response when exposed to disease; they may be especially vulnerable to disease outbreaks."
The last active case of measles in Washington was reported in late April.
It's possible to develop measles within three weeks of exposure. Since three weeks have passed since the last measles case, no one who had contact with the known cases is at risk, Moyer said.
This case was not related to the measles outbreak at Disneyland, which started in December and sickened more than 140 people across the country and in Mexico and Canada, Moyer said. No deaths resulted from that outbreak.