LOS ANGELES (AP) - County health officials warned Thursday of a marked increase in cases of whooping cough, the highly contagious disease that can cause weeks of severe coughing.

In Los Angeles County, there have been 108 suspected cases of the disease, also called pertussis, so far this year, with infants making up most of those infected.

The number is a 50 percent increase over the same period last year, said Dr. Jonathan Fielding, director of public health and health officer for Los Angeles County. Similar increases have been noted elsewhere across the state and nation.

Traditionally, doctors have regarded pertussis as an infant disease, but it is making a global comeback in all age groups.

Scientists suspect protection from immunization wears off after a few years and that the bug has outsmarted the vaccines that have controlled it for decades. Growing numbers of unimmunized children may also play a role in its comeback.

Whooping cough is caused by the bacteria Bordetella pertussis. It is usually mild in adults but can be fatal in infants. It kills between five and 10 children in the United States each year, according to the Centers for Disease Control and Prevention.

``It is important for parents in general to try to keep people who are coughing away from their kids,'' Fielding said.

In most countries, infants get their first pertussis shot at 1 month old. They are immunized again at 2 months, then 3 months, and given a booster at 15 months.

Most people with the disease have a cough that lasts more than three weeks. It can include night coughing that disturbs sleep, vomiting, 30-second sweating attacks and complications such as hernias or rib fractures.

Health officials urge prompt reporting of pertussis cases to prevent its spread. At early stages, the disease is readily treatable with antibiotics.

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On the Net: Pertussis fact sheet http://www.cdc.gov/ncidod/dbmd/diseaseinfo/pertussis--t.htm