There have been nearly 1,500 confirmed cases, the most since the 1950s.
Infants are the most vulnerable and can end up becoming severely ill from a disease that mimics the common cold for weeks.
Doctors recommend vaccinations at ages two months, four months, six months, and a booster at about three and a half years old.
"They'll cough, cough, cough and keep coughing. Air goes out, but doesn't come back in and so they basically suffocate. They turn blue and then sometimes they stop breathing," says Dr. Dean Blumfeld, M.D. from U.C. Davis.
Out of the nearly 1,500 cases this year in California, five have been fatal. All were Latino infants under three months of age. Whooping cough cases peak in July, Aug., and Sept.
The Contra Costa County Health Services Department will hold a free whooping cough vaccination clinic in Concord this Thursday. The clinic will take place from 4 to 9 p.m. at the music and market event at Todos Santos Plaza at the Concord Farmers Market.