Federal health officials are investigating an April data breach that affected 55,947 patients of Dignity Health, a major health system headquartered in San Francisco that operates 39 hospitals and 400 care centers in California, Nevada and Arizona. The breach occurred April 24 and was reported to the U.S. Department of Health and Human Services on May 31. It was the third-largest data breach, by number of affected patients, reported to the federal health agency that month. The incident is being reviewed by the department’s Office for Civil Rights, which investigates breaches of protected health information that affect at least 500 people. The office did not return a request for comment Tuesday.