Hints that a retroviral infection might play a role in chronic fatigue syndrome have been present from the beginning. In 1991, Dr. Elaine DeFreitas, a virologist at the Wistar Institute in Philadelphia, found retroviral DNA in 80 percent of 30 chronic fatigue patients. The C.D.C. went so far as to try to replicate her effort, but refused to follow her exacting methods for finding the virus. In addition, the centers’ blood samples became contaminated, and some people at the agency said that administrators ended the research prematurely. Rather than admit any such failure, the C.D.C. publicly criticized Dr. DeFreitas’s findings.