Human parvovirus B19, an erythrovirus, causes fifth disease. It is transmitted from person to person either by respiratory secretions or droplets or through blood products; it is also transmitted from mother to fetus during pregnancy. Fifth disease is contagious in the early stages, before the rash appears. It has an incubation period of 4 to 14 days. Epidemics of fifth disease occur in the late winter or early spring; during school outbreaks, 10% to 60% of students may be infected.

Veterinarians are concerned about a type of parvovirus that affects pets. This is not the same virus, and fifth disease cannot be transmitted from pets to humans or from humans to pets.

Fifth disease can cause aplastic crisis in patients with a history of sickle cell anemia, thalassemia, autoimmune hemolytic anemia, or pyruvate kinase deficiency. Chronic bone marrow failure may occur in patients with a history of human immunodeficiency virus infection, acute lymphocytic leukemia, and congenital immunodeficiency syndromes. If a woman is infected during pregnancy (usually during the first half of pregnancy), fifth disease may lead to severe anemia or congestive heart failure in the mother and hydrops fetalis (fetal anemia) or intrauterine death of the fetus, although intrauterine death occurs in less than 5% of cases. Other complications that may be triggered by fifth disease include rheumatoid arthritis, idiopathic thrombocytopenic purpura, myocarditis, hepatitis, uveitis, glomerulonephritis, and seizures.

Approximately 20% of patients with fifth disease are asymptomatic. Fifth disease mostly affects the skin and joints. A few days before the rash appears, the patient may complain of coldlike symptoms (fever, sore throat, muscle aches, and nasal congestion). Patients will present with a classic “slapped-cheek” redness on the cheeks that resembles sunburn. This rash may fade within 2 to 4 days. Another macular rash appears on the extremities 1 to 4 days after the initial cheek redness but does not usually affect the palms of the hands or soles of the feet. This rash fades into a lacy pattern that may last up to 3 weeks. It is usually not itchy, but some itching may occur. (See Fifth disease rashes.)