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.
During this early stage, the virus is present in the blood, and the person is infectious to others. The anemia develops during this stage, but since children often do not have symptoms, no one checks for it. By this time, the person has made antibodies to B19, and the virus is no longer present in the blood. It is at this point that children develop the typical slapped face rash, which often spreads to the trunk and arms. Susceptible adults may or may not get a rash, but often develop an arthritis which can last for a couple of weeks.