The head of the U.S. Centers for Disease Control and Prevention tested positive for COVID-19 a second time in almost as many weeks after contracting the virus and isolating earlier this month, the federal agency said Monday morning.Dr. Rochelle Walensky, a former Massachusetts General Hospital doctor, began developing symptoms Sunday and “has again tested positive,” the CDC said in a statement. The test result comes after the agency announced on Oct. 22 that she was experiencing mild symptoms and tested positive.