fifth disease

WordReference Random House Unabridged Dictionary of American English © 2025

fifth′ disease′, 

1. Pathology a mild infection, most often seen in children or young adults, caused by a small virus  (the human parvovirus B19 )  and marked by a blotchy rash on the cheeks, arms, and legs.
Also called erythema infectiosum. 
• so called because it was the fifth in a classification scheme of childhood eruptive diseases