My toddler has a fever (102-104.5) for 3 days, then a rash. what could this be?

Roseola: is a common viral infection (one of the herpes viruses) that affects young children, and is characterized by a very high fever lasting usually 2-4 days usually associated with URI symptoms and fussiness, followed by a rash which develops to the body when the fever resolves. Treatment is symptomatic and complications are rare. Immunity is believed to be life long.