As of June 30, 1,337 cases of the highly contagious disease were reported from across the state, and 700 more cases remained under investigation, said Ken August, a spokesman for the California Department of Public Health.
Whooping cough, also known as pertussis, was declared an epidemic in California last month after 910 cases of the highly contagious disease were reported as of June 15, and 600 more cases were under investigation.
Even after 19 days in the hospital, 2-month-old Jose Sanchez still has trouble breathing. Baby Jose is one of 38 whooping cough cases admitted into Childrens Hospital Los Angeles in the last six months, most of them in of June.
"They can get extremely sick, be in the hospital a long time and occasionally succumb - even healthy, previously healthy children," said Dr. Lawrence Ross, a pediatrician at Childrens Hospital Los Angeles.
Five California babies under three months of age have died of the illness. Eighty-nine percent of cases are kids less than a year old. Unimmunized or incompletely immunized babies are particularly vulnerable.
A typical case starts with a cough and runny nose for one to two weeks, followed by weeks or months of rapid coughing fits that sometimes end with a whooping sound. Fever is rare.
What's scary about whooping cough is that even though you get an antibiotic and you're being treated for it, infectious disease experts say it can still be deadly.
"They can have more brain injury, they can have problems with their heart, and problems with their kidneys," said Dr. Ross.
Just like a cold or flu, the whopping cough bacteria is transmitted through close contact and droplets so hand washing is a must. But Dr. Ross says getting vaccinated is the best protection for both adults and kids.
"Even if you've had whooping cough as a child after years you're at risk at getting it again as well, so neither immunity from the infection nor from the vaccination is lifelong and you need to be re-immunized," said Dr. Ross.
"When a child is not vaccinated that puts another person out there who can not only get infected but can get infected and then transmit it to somebody else," he added.
Dr. Ross says the protection from the vaccine usually lasts about five to seven years, so it's very important for adults and the elderly to get boosters.
The number of cases in the state represents more than a five-fold increase over the same period last year, when 258 cases were reported. But the new numbers may not represent a current surge in infection because the increase includes cases that are months-old.
The number of infections is expected to peak in August and early September.
The Associated Press contributed to this report.