SAN FRANCISCO  After the deaths of five infants, California health authorities declared an epidemic of whooping cough in the state on Wednesday, urging residents  particularly those of Latino background  to get vaccinated against the disease.

The announcement came after authorities noticed a sharp spike in reports of pertussis, the scientific name for whooping cough, which often is mistaken for a cold or the flu and is highly contagious. All told, 910 cases have been confirmed, with several hundred more under investigation. If the pace keeps up, the outbreak could be the largest in the state in 50 years, the California Department of Public Health reported.

Dr. Gilberto Chavez, the deputy director of the department’s Center for Infectious Disease, said health officials had seen a fourfold increase compared with 2009. And the worst may be to come.

“The peak season starts in the summer,” Dr. Chavez said, noting that July and August usually have the highest number of cases. “And we expect to see a much larger number of cases if we don’t intervene quickly.”