Sixth disease is caused by human herpes virus-6B.  It is also called Exanthem subitum, Roseola infantum, Sudden Rash, rose rash of infants, 3-day fever. This a common disease of young children from 6 months to 18 months (in the US >45% of children are seropositive  for HHV-6 by two years of age) and symptoms include fever and sometimes upper respiratory tract infection and lymphadenopathy. The symptoms last a few days after an incubation period of around 14 days. The fever subsides leaving a macropapular rash on the trunk and neck that last a few days longer. In adults, primary infection is associated with a mononucleosis. This virus was originally isolated from patients with a lymphoproliferative disease and may co-infect HIV-infected T4 lymphocytes exacerbating the replication of HIV. Patients with HIV have a higher infection rate than the normal population.   HHV-6 has  been associated with a number of neurological disorders, including encephalitis and seizures. It has been postulated to play a role in  multiple sclerosis and chronic fatigue immunodeficiency syndrome.