LOS ANGELES (AP) — More than 5,270 cases of whooping cough have been reported in California’s growing epidemic, which has killed nine babies this year.

This week’s report from the California Department of Public Health finds that the highly contagious illness hasn’t infected this many in the state since 1955, when 4,949 cases were reported for the entire year.

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Whooping cough is a cyclical illness that peaks in number of infections every five years. Symptoms are similar to the common cold, making it a challenge to diagnose. A persistent cough that lasts weeks is the tell-tale symptom of the illness, which is also known as pertussis.

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The bacterial infection tends to peak during summer months, but reporting lags make it difficult to determine if the peak has passed.

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