Khan, A S, Heneine, W M, Chapman, L E et al. · Annals of internal medicine · 1993 · DOI
Researchers tested whether a virus called HTLV-II might be a cause of ME/CFS by comparing blood samples from 21 people with CFS to 21 healthy controls. They found no trace of the HTLV-II virus in any of the participants, and found no differences between the two groups in exposure to activities that spread viruses. This study argues against the idea that HTLV-II is linked to ME/CFS.
In the early 1990s, many hypotheses about ME/CFS etiology were proposed, including retroviral infection. This study helped rule out HTLV-II as a primary factor, narrowing the field of investigation and preventing resources from being spent on less promising leads. Clarifying what does not cause ME/CFS is important for redirecting research toward more plausible mechanisms.
This study does not prove that no retrovirus plays any role in ME/CFS—it specifically rules out HTLV-II and common retroviral transmission routes in this small population. It does not exclude other viruses (such as EBV, HHV-6, or enteroviruses) that were investigated in later research. The small sample size limits generalizability and the study's statistical power to detect rare associations.
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
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