Complex sentence: Early medical observations suggested that CFS cases commonly presented with an acute infection-like onset and acute onset has been reported by researchers to be more common in CFS patients than in those patient suffering from simple chronic fatigue.
Split into simpler sentences: Early medical observations suggested that CFS cases commonly presented with an acute, infection-like onset. Acute onset has been reported by researchers to be more common in people with CFS than in people only experiencing chronic fatigue.