Complex sentence: The cause of erythema toxicum is unknown, but it is thought to be a benign condition that causes no discomfort to the infant and the rash will generally disappear spontaneously in about 2 weeks.
Split into simpler sentences: The cause of erythema toxicum is unknown, but it is thought to be a benign condition that causes no discomfort to the infant. The rash will generally disappear spontaneously in about 2 weeks.