What is ME/CFS?

Myalgic encephalomyelitis, or chronic fatigue syndrome, usually begins with an ordinary infection from which the patient never recovers. Its signature is post-exertional malaise: activity that would once have been trivial triggers a delayed crash of exhaustion, pain, and cognitive fog lasting days. The name dates to a 1956 hospital outbreak in London, and the condition affects tens of millions worldwide, a quarter of them housebound or bedbound. For decades it was widely dismissed as psychological, a verdict now recognized as one of modern medicine’s costlier wrong turns. What it actually is remains unsolved.

Why it matters

The scale alone is enormous: ME/CFS disables more people than many famous diseases, at vast economic cost, with patients commonly ill for decades. The question gained urgency when COVID-19 minted millions of new cases of long COVID, a substantial fraction of which meet ME/CFS criteria, making the mystery suddenly current for health systems everywhere. And the illness poses a sharp scientific puzzle in its own right: what stable, long-lasting switch can an infection flip in a body that standard tests then read as normal?

What has been tried

The field carries scars: a celebrated 2009 retrovirus finding collapsed in retraction, and a major trial of exercise and behavioral therapy was influential for years before its methods and message were widely criticized, with graded exercise now dropped from major guidelines because exertion worsens the core symptom. Real findings have accumulated since: measurable abnormalities in immune signaling, in the gut microbiome, in autonomic control of heart rate and blood pressure, and in cellular energy metabolism, where studies point to impaired oxygen use under repeat exertion. Two-day exercise testing shows an objective performance drop on the second day that healthy controls do not show. A deep-phenotyping study by the US National Institutes of Health found immune and brain differences in a small, carefully studied cohort. What is missing is convergence: no finding yet replicates cleanly enough to diagnose an individual patient.

Where the edge is

Competing mechanistic pictures, viral persistence, autoimmunity, metabolic trapping, neuroinflammation, are not mutually exclusive and none is established. The field lacks its keystone: a validated biomarker, which would at once confirm the biology, enable trials, and end the argument about legitimacy that patients have carried for seventy years.

What would count as an answer

A replicated diagnostic marker, a mechanism explaining post-exertional malaise, and a treatment that works because the mechanism says it should. Any one of the three would move this entry; all three would retire it.