On Apr. 3, 2012, the Washington State Secretary of Health declared a pertussis epidemic. By Jun. 16, the reported number of cases had reached 2,520 (37.5 cases per 100,000 residents), a nearly 1,300% increase compared with the same period in 2011 and the highest number of cases reported in any year since 1942.

Whooping cough is a respiratory illness caused by a type of bacteria called Bordetella pertussis. Whooping cough bacteria attach to the cilia (tiny, hair-like extensions) that line part of the upper respiratory system. The bacteria release toxins (poisons), which damage the cilia and cause airways to swell.