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Roseola (roseola infantum) is a mild illness caused by a virus. It is generally harmless and is most common in children 6 months to 2 years of age. It is rare after age 4. Roseola is caused by two common viruses.
1 Roseola is a mild viral illness most commonly involving young children. 2  A sudden high fever that lasts for three to five days is an early feature of roseola. 3  Mild nasal congestion and loose stools may accompany the fever. 4  When the fever disappears, a rash appears, which may last one to two days.