Fifth disease (also known as erythema infectiosum) is caused by a virus called parvovirus B19. (In the pre-vaccination era, "fifth disease" was called as such because it was often literally the fifth disease that a child contracted).
Symptoms include low-grade fever, fatigue, a "slapped cheeks rash," and a rash over the whole body.
Parvovirus B19 can temporarily decrease or halt the body's production of red blood cells, causing anemia.
Pregnant women who have not previously had the illness should avoid contact with people who have fifth disease. The fifth disease virus can infect a fetus. And, while no birth defects have been reported as a result of fifth disease, it can cause the death of the unborn fetus. The risk of fetal death is five to 10 percent if the mother becomes infected.