Human parvovirus B19 capsid, molecular model. This is the capsid (outer protein coat) of the parvovirus B19, best known for causing erythema infectuosum or Fifth disease in children. This disease causes an unusual redness on the face, earning it the name 'slapped cheek disease.' In adults, Fifth disease is more serious, causing swelling and joint pain. Parvoviruses are small and consist of a single-stranded DNA genome and an icosahedral capsid. The solid colours represent the secondary structure of the protein (spiral ribbons are alpha helices; arrows are beta sheets), and the faded tubular structure behind represents the individual atoms and bonds.