A childhood disease or childhood ailment was a type of medical condition that usually affected children, although they could be contracted in adulthood as well, much to the patient's embarrassment. (TNG: "Up The Long Ladder")
Examples of such diseases included measles (Humans), rop'ngor (Klingons), and Orkett's disease (Bajorans). (TNG: "Up The Long Ladder"; VOY: "State of Flux")