Post-exertional malaise (PEM) is the hallmark symptom of ME/CFS: an exacerbation of symptoms triggered by physical or mental exertion that is often disproportionate to the activity. While PEM has long been recognized as central to ME/CFS, the molecular mechanisms underlying this debilitating response have remained largely unknown.

The study found that key CD8+ T cells showed signs of constant stimulation that lead to an exhausted state, a condition that is well-studied in cancer.