 The review compares the symptoms of myalgic encephalomyelitis slash chronic fatigue syndrome, me slash CFS, and long COVID, finding them to be very similar. It also examines the underlying causes of the two diseases, focusing on abnormalities of the central and autonomic nervous system, lungs, heart, vasculature, immune system, gut microbiome, energy metabolism, and redox balance. This comparison reveals the strength of the evidence for each abnormality in each disease, helping to set priorities for further research. This article was authored by Anthony L. Komaroff and W. E. N. Lipkin.