B19 virus is most known for causing disease in the pediatric population; however, it can also affect adults. It is the classic cause of the childhood rash called fifth disease or erythema infectiosum, or "slapped cheek syndrome." Primate erythroparvovirus 1, generally referred to as B19 virus, parvovirus B19 or sometimes erythrovirus B19, was the first (and until 2005 the only) known human virus in the family Parvoviridae, genus Erythroparvovirus; it measures only 23-26 nm in diameter.