B19 virus, a strain of human parvovirus associated with a number of diseases, including hemolytic anemia, erythema infectiosum, fifth disease, and symptoms of arthritis and arthralgia. B19 infects only humans. Approximately 50% of adults have been infected some time during childhood or adolescence. Children infected with erythema infectiosum, the most common illness caused by B19, develop a mild rash, usually across the face, which usually resolves in 7 to 10 days. Postinfection children develop lasting immunity. Infection in adults not previously infected with B19 is usually more severe, involving joint aches and swelling, most often resolving in 2 to 3 weeks.
