Fifth disease is the common childhood exanthem also known as erythema infectiosum and is caused by parvovirus B19. (Everyone knows that.) But what is fourth disease?

It was a childhood exanthem described around 1900 that has been abandoned as a nosologic entity. A century ago, common childhood exanthems were assigned numbers one through six for ease of classification. Only fifth disease retains its numeric name. Fourth disease, also called Dukes’ disease or Filatow-Dukes disease, purportedly consisted of a nonspecific febrile illness accompanied by a cutaneous eruption. It may have been a streptococcal toxin–mediated disease, but it is not considered a precise or specific disease description.

Bialecki C, Feder HM, Grant-Kels JM: The six classic childhood exanthems: a review and update, J Am Acad Dermatol 21:891–903, 1989.