Increasing incidence of whooping cough is ringing alarm bells across the State of California even as the infection continues to spread, published media reports said.

In what is considered the largest outbreak of this highly contagious disease over the past 50 years, public health officials in California are urging residents, particularly pregnant women and those who come into contact with infants, to get immunized as soon as possible.

Medically known as pertussis, whooping cough is characterized with the wheezing sound, or "whoop," sufferers make when they cough and control their breath.

Whooping cough can normally last for weeks in adults. The bacterial infection can be treated with antibiotics and rarely life-threatening. But whooping cough can be deadly in infants too young to be immunized.

More than two-thirds of infants who get pertussis will be hospitalized. About one in 10 children infected with the disease goes on to develop pneumonia, while one in 250 get the disease that affects the brain, called encephalopathy, according to the Centres for Disease Control.

Around 1,500 cases of whooping cough have been reported in California in the first half of the year, compared to a little more than 300 cases in the first part of 2009, according to the California Department of Public Health.

"The magnitude of the increase in California is concerning, and that's why we are trying to emphasize the importance of vaccination," said Jennifer Liang, an epidemiologist with the U.S. Centers for Disease Control and Prevention.

"Pertussis is a cyclic disease, and we do see peaks every three to five years," Liang said. "The last peak was 2005, when we had 25,000 reported cases nationally, and we may be on the upswing of another cycle."

Pertussis is usually prevented through vaccinations for children at two months, four month, six months, between 15 and 18 months and then again at school age, between four to six years. Children should also receive a booster between 11 and 12 years.

The CDC has also recommended adolescents and adults up to age 64 receive a one-time booster shot because immunity from childhood pertussis immunization declines over time. The whooping cough vaccine called Tdap also includes a tetanus and diphtheria booster.