Fifth disease is caused by human parvovirus B19. It often affects preschoolers or school-age children during the spring. The disease is spread by contact with respiratory secretions and usually lasts for five days. However, the rash associated with fifth disease may recur for several weeks afterwards. Recurrences may be brought on by exposure to sunlight, heat, exercise, fever, or emotional stress.

The first sign of the disease noticed by parents is usually bright red cheeks, which look as though the child has been recently slapped on both sides of the face. Following this, a rash appears on the extremities and trunk. The rash fades from the center outwards, giving it a lacy appearance. Over a period of 1-2 weeks, the rash disappears entirely. This illness is also sometimes associated with fever.

Infection of a pregnant woman can harm the fetus. Although the risk of fetal harm is small, if a pregnant woman believes that she may have been in contact with a person who has parvovirus, she should consult with her physician.

Parvovirus B19 is also thought to cause other disease manifestations. In healthy adults (particularly women), it may be responsible for an infectious form of arthritis. In people with AIDS (or others with weakened immune systems), it may produce a profound anemia (deficiency of certain blood cells), called 'transient aplastic crisis'.

The incidence of parvovirus B19 infection is unknown, but studies suggest that a majority of adults have antibodies to the virus in their bodies. This indicates that most people have been exposed to the virus, and also suggests that many infections go unnoticed.

• Rash, first appearing on the cheeks, often looks like "slapped cheeks"
• A rash spreading to the extremities about one day later, often has a "lacy" appearance
• Fever
• Joint pain also called arthralgia

The patient is examined for the appearance and pattern of the rash. A classic appearance of the rash may make the diagnosis straightforward.

Blood tests for antibodies against parvovirus B19, which may indicate infection, are available, although they are not commonly necessary. They may be of use in the diagnosis of "aplastic crisis" and persistent anemia. Testing may also be done when there is a question of exposure in a pregnant mother whose immune status is not known.