Over 1,000 current and former employees of the Department of Health and Human Services have demanded that Secretary Robert F. Kennedy Jr. resign for “placing the health of all Americans at risk.”    The signatories, who released the letter on Wednesday, include representatives from a number of health-related agencies, such as the Administration for Children and Families, the Centers for Disease Control and Prevention, the National Institutes of Health, the Centers for Medicare and Medicaid, and