On Apr. 3, 2012, the Washington State Secretary of Health declared a pertussis epidemic. U.S.  Since mid-2011, a substantial rise in pertussis cases had been reported in the state of Washington.  By June 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. 

Consistent with national trends, high rates of pertussis were observed among infants aged <1 year and children aged 10 years. However, the incidence in adolescents aged 13–14 years also was increased, despite high rates of vaccination with tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis (Tdap) vaccine, suggesting early waning of immunity. The focus of prevention and control efforts is the protection of infants and others at greatest risk for severe disease and improving vaccination coverage in adolescents and adults, especially those who are pregnant. Pertussis vaccination remains the single most effective strategy for prevention of infection.