Whooping cough, also known as pertussis, can be deadly, especially in babies. And a new study finds that even though babies are can be vaccinated against pertussis, the incidence is climbing.

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"The case was less than 700 per year in the 1970s, but the number has increased to more than 2,000 per year at the end of the 1990s," said Dr. Mashiro Tanaka, of the Centers for Disease Control and Prevention.

Tanaka and his colleagues have been tracking whooping cough cases for several years.

They studied more than 30,000 cases that occurred between 1980 and 1999. The findings, published in Wednesday's issue of the Journal of the American Medical Association, show an increase of nearly 50 percent in the number of new cases.

"Almost all the increase was among very young infants younger than 5 months old. These infants were too young to be fully vaccinated against whooping cough," Tanaka said.

Pediatricians recommend babies receive the pertussis vaccine at 2, 4 and 6 months of age to be fully protected against whooping cough. Infants younger than 6 months are at special risk.

"We think that many infant cases were exposed to people who had unrecognized illness or mild whooping cough," Tanaka said.

Tanaka recommends that parents keep their infants away from anyone who has a consistent cough. Even if it's not a violent cough it could be whooping cough.

And parents should make sure infants are immunized against whooping cough on schedule.