The bacterial infection causes uncontrollable, violent coughing, which often makes it hard to breathe. People often take deep breaths which result in a "whooping" sound.
San Diego County is among the hardest hit areas with 1,819 cases reported so far this year.
"We'd have to go way back to the 1940s to find more cases," said Dr. Eric McDonald, medical director of the epidemiology and immunization branch for public health services in San Diego County.
That's when whooping cough was common, causing hundreds of thousands of illnesses annually and thousands of deaths. But after a vaccine was introduced in the 1940s, cases dropped to fewer than 5,000 a year.
That vaccine was replaced in the 1990s because of side effects, which included pain and swelling from the shot and fever. The newer vaccine is part of routine childhood vaccinations as well as adult booster shots.
Last year was the nation's worst year for whooping cough in six decades— U.S. health officials received reports of more than 48,000 cases, including 18 deaths. This year the number of reported cases nationwide dropped to about 20,000.
After the 2010 epidemic, California launched a campaign about the importance of rapid diagnosis and treatment, especially in young infants. The state also started providing free vaccines for children, pregnant and postpartum women.
Dr. Gil Chavez, epidemiologist with the California Department of Public Health, said while more people, especially pregnant women, need to get vaccinated, he does not believe low inoculation rates are the primary cause of the current epidemic: Of this year's pediatric cases that had information on the child's vaccination history, only 10 percent of those infected in 2014 had not been vaccinated against pertussis.
Chavez says the new vaccine's limitations and better tests have led to the increase in cases.
Cases are likely to continue going up as doctors do a better job at detecting the illness, officials say.
More than two years ago, Kathryn Riffenburg, who lives outside Boston, said doctors told her that her newborn son, Brady, likely had a cold. A week and a half later, she took him to the emergency room as he struggled to breathe.
By the time, he was diagnosed with pertussis it was too late to save the 2-month-old boy.
"It made us angry, because we felt more should have been done," said Riffenburg, who know advocates for pregnant women and anyone else in contact with infants to get vaccinated.