An infant died earlier in December from pertussis, which is commonly known as whooping cough, said officials with the King County Public Health Department.
In 2012, there were 752 confirmed cases of pertussis reported among King County residents, the highest number in more than a decade.
Infants are the highest risk for serious illness, hospitalization and death from whooping cough.
Officials urge mothers and family members should be immunized to protect newborns and young babies.
The King County Public Health Department has more more information on pertussis and how to get vaccinated.