Nearly 1,500 Californians this year have been diagnosed with whooping cough — five times the normal level for this time of year, state health officials say. Doctors are investigating another 700 possible cases. Many more may have had the infection, which often goes undiagnosed or unreported.

"This has the potential to become very huge," says Gilberto Chavez, chief of the California Department of Public Health's Center for Infectious Disease. "But we are at a point where we can contain it."

Chavez says it's especially important to protect babies, who have no natural immunity to whooping cough, also known as pertussis, and who are the most likely to die from it. Although infants can get their first shots at 6 weeks, they aren't fully protected until after their third shot, at 6 months. Nearly 70% of infants under age 1 with whooping cough are hospitalized, according to the Centers for Disease Control and Prevention.

Whooping cough cases have been increasing since the 1980s, partly because of better diagnostic tests, according to the CDC. It's incredibly contagious, sickening about 90% of people who are exposed to it.

The only way to protect babies — who typically contract whooping cough from their families — is to vaccinate everyone around them — parents, grandparents, siblings, caregivers, neighbors and friends, says Stacey Martin of the CDC. High vaccination rates with create a "herd immunity" for the entire community, even those who are unvaccinated, Martin says.

The state health department is providing whooping cough boosters to new mothers and other close contacts of infants at all birthing hospitals, community health centers and local health departments.