Fifth disease is a viral illness caused by parvovirus B19. Fifth disease is also known as "erythema infectiosum" and "slapped cheek disease." The clinical illness was described in the 1880s and was named fifth disease because of its fifth position in the numerical classification of childhood illnesses associated with rashes (exanthems). Other numbered diseases included measles, scarlet fever, andrubella -- and these did not get renamed until the molecular era, when it became possible to isolate viruses and bacteria. Fifth disease was number five of six diseases named. It is a very common infection, and almost 50% of adults have been infected with the virus but do not remember having it because it often does not cause symptoms.