There is no evidence to support the view that CFS is a contagious disease. Contagious diseases typically occur in well-defined clusters, otherwise known as outbreaks or epidemics. While some earlier studies have been cited as evidence for CFS acting as a contagious illness, they did not rigorously document the occurrence of person-to-person transmission. In addition, none of these studies included patients with clinically evaluated fatigue that fit the CFS case definition; therefore, these clusters of cases cannot be construed as outbreaks of CFS. Implicit in any contagious illness is an infectious cause for the disease.

Carefully designed case-control studies involving rigorously classified CFS patients and controls have found no association between CFS and a large number of human disease agents. Finally, none of the behavioral characteristics typically associated with contagious disease, such as intravenous drug use, exposure to animals, occupational or travel history, or sexual behavior, have been associated with CFS in case-control studies. It therefore seems unlikely that CFS is a transmissible disease. Nevertheless, the lack of evidence for clustering of CFS, the absence of associations between specific behavioral characteristics and CFS, and the failure to detect evidence of infection more commonly in CFS patients than in controls do not rule out the possibility that infectious agents are involved in or reflect the development of this illness. For example, important questions remain to be answered concerning possible reactivation of latent viruses (such as human herpesviruses) and a possible role for infectious agents in some cases of CFS. 

Clinical Course of CFS