Wiktionary
fifth disease
n. (context disease English) The manifestation as rashes of an infection by erythrovirus.
Wikipedia
Fifth disease
Not to be confused with sixth disease. See Exanthema subitum.
Erythema infectiosum or fifth disease is one of several possible manifestations of infection by parvovirus B19. The disease is also referred to as slapped cheek syndrome, slapcheek, slap face or slapped face.
The name "fifth disease" comes from its place on the standard list of rash-causing childhood diseases, which also includes measles (1st), scarlet fever (2nd), rubella (3rd), and Dukes' disease (4th), though the last is no longer widely accepted as distinct.