It is called Fifth disease because in an old classification of childhhood diseases that cause a rash, it was the fifth disease listed. The virus causing the disease is called B19.
Infection occurs most commonly as school outbreaks during the winter and spring.
About 50-60 percent of adults have antibodies to the disease, meaning that they probably had it as children and are now immune. The virus primarily infects the bone marrow cells that produce red blood cells, and it often causes a brief anemia, but this is rarely recognized unless it occurs in someone who is already severely anemic, like a child with sickle cell disease or thalessemia. The virus simply can't get into their blood cells. Infection probably occurs through contact with nasal secretions or cough droplets shed by an infected child. Children often have no symptoms.