Parvovirus B19 is the cause of erythema infectiosum (fifth disease) in young children. The infection is characterised by fever and rash with erythema of the cheeks (“slapped cheek' appearance); there is usually a lacy pink or morbilliform rash on the limbs and trunk. Parvovirus is particularly associated with the production of arthropathy; this is much more frequent in adults, especially women, in whom primary parvovirus B19 infection can produce polyarthritis, in about half of cases without erythema infectiosum. Parvovirus B19 is tropic for erythroid progenitor cells (the erythrocyte P antigen is the virus receptor). Infection causes erythroid arrest, which can result in aplastic crises in individuals with sickle-cell disease, and other diseases with high red-cell turnover. Intrauterine infection from infection in pregnancy can result in hydrops fetalis, due to fetal anaemia from infection of fetal erythroid progenitors. Infection in immunodeficient subjects can cause chronic anaemia associated with persistent B19 infection; the virus can be cleared in these patients by administration of intravenous immunoglobulin (which contains neutralising antibody). Diagnosis is by detection of IgM antibody to B 19, or of virus DNA in blood.