Sixth disease: A viral disease of infants and young children with sudden onset of high fever which lasts several days and then suddenly subsides leaving in its wake a fine red rash. The causative agent is herpesvirus type 6 so the disease is known as Sixth Disease. Also known as Exanthem subitum (sudden rash), Pseudorubella, Roseola, Roseola infantilis, and Roseola infantum.