Incorrect. The answer was Varicella.
You were on the right path with your train of thought. You discovered it was a diffuse rash, childhood illness, caused by an airborne virus, and was self limiting. Those are all true ideas for measles and also varicella (chicken pox)
Had you inquired in greater detail about severity of the illness, additional manifestations, fatigue, body aches, or chance of recurrence you would have likely arrived at measles;
Measles: people feel sick for a longer time and suffer from fever, fatigue and achiness as well as coughing, runny nose, and watery eyes.