Fifth disease is caused by a virus called human parvovirus B19. The illness causes the body to stop making red blood cells for a short time. This usually does not cause a problem for an otherwise normally healthy child or adult. But it can become a serious threat for certain people.

Pregnant women

If a woman gets fifth disease during pregnancy and she's never had the infection before, the doctor will want to watch for certain problems in the fetus. In very rare cases, a fetus that becomes infected with parvovirus B19 may develop severe anemia and swelling, a condition called fetal hydrops. The mother and fetus should be closely watched with fetal ultrasounds to detect this condition.

When fetal hydrops is detected, the fetus may be treated with blood transfusions while in the uterus, although this is not usually needed.

Some babies born to mothers who were infected with fifth disease during pregnancy may also be treated with blood transfusions.

People who have blood or immune disorders

People who have blood disorders that cause anemia (such as sickle cell disease or thalassemia) may require blood transfusions if a rapid worsening of existing anemia (called transient aplastic anemia) develops. People who have this type of anemia can become very sick. Symptoms include fever, lethargy, rapid heart rate, and rapid breathing.

People who have impaired immune systems and get fifth disease may develop a chronic parvovirus B19 infection that can lead to severe anemia. Intravenous immunoglobulin (IVIG) may be needed to prevent a chronic parvovirus B19 infection and severe anemia.