Complex sentence: Although early reports described epidemics, and by the 1950's several tens had occurred worldwide (see ME/CFS outbreaks), it was established that the disorder was primarily found among the general population and the epidemic form was the exception.
Split into simpler sentences: Although early reports described epidemics, and by the 1950's several tens had occurred worldwide. It was established that the disorder was primarily found among the general population and the epidemic form was the exception.