Roseola is the most common infection to cause a rash after a fever. Roseola is a viral infection. This is a common infection for infants and toddlers. It usually causes a high fever that goes up and down for 3 days. About the time the fever breaks a rash of red spots appears on the trunk and spreads to other parts of the body. The rash lasts for a few days and then goes away. Other viruses can also cause a rash after a fever.