The long-term illnesses that can follow viral infections can be devastating — and are devastatingly common. In 2015, the nation’s top medical advisory body, the Institute of Medicine, estimated that between 800,000 and 2.5 million U.S. residents live with the illness or illnesses awkwardly named myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). An estimated three-quarters of these cases were triggered by viral or bacterial infections.