My daughter had a high fever for 3 days then after fever left she got a rash all over. what could cause this?

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.