Whooping cough, also called pertussis, is a bacterial infection that causes severe coughing. It can be difficult to diagnose because symptoms are similar to a cold.
With the ninth death in the state, health officials say that's more than those who died from the illness in 2005.
In 2010 alone, 3,600 cases of whooping cough have been reported in California. That number is a seven-fold increase from 2009 and the most since 1958.
The Associated Press contributed to this report.
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