For decades, the Centers for Disease Control and Prevention was the undisputed global leader in public health. Its scientists helped train health officials worldwide. In the United States, it was the primary source of guidance for prevention and treatment of diseases, including diabetes, H.I.V. and the seasonal flu.

But after a year and a half in the cross hairs of Health Secretary Robert F. Kennedy Jr.’s drive to remake public health policy, the agency is a skeleton of the powerhouse it was.

The Trump administration’s widespread layoffs in early 2025 cut 18 percent of the C.D.C.’s staff. But new personnel data, compiled by a group of former federal scientists and reported here for the first time, indicate that the agency’s staff has dwindled far more, as large numbers of employees retired or resigned. As of May 2026, the C.D.C. had 30 percent fewer employees than it did before the layoffs, the data show.

Interviews with more than 30 current and former employees reveal the agency is now tightly controlled by Mr. Kennedy and his associates, and its scientists are finding it increasingly difficult to do their jobs effectively. (Some of them spoke on the condition of anonymity because of a fear of retaliation from the administration.)