Sixth Disease Sixth Disease
Wikipedia–The Free Dictionary

Exanthema subitum (meaning sudden rash), also referred to as roseola infantum (or rose rash of infants), sixth disease (as the sixth rash-causing childhood disease) and (confusingly) baby measles, or three-day fever, is a disease of children, generally under two years old,[1] whose manifestations are usually limited to a transient rash ("exanthem") that occurs following a fever of about three day’s duration.

It is frequently called roseola, although this term could be applied to any rose-colored rash