Parvovirus B19 is the infectious agent responsible for an illness called “fifth disease.” This test is used to assess the presence of and immunity from parvovirus B19 (e.g., in pregnant women). While anti-parvovirus IgM develops one to two weeks after initial exposure, IgG appears a few weeks later and increases during the active phase of infection and stabilizes after recovery. Once a person has been exposed to parvovirus B19, they will have detectable levels of IgG antibodies in their blood for the rest of their life. The parvovirus B19 IgG test may be ordered along with the parvovirus B19 IgM test. If IgG and IgM results are positive, the tested person may have an active parvovirus B19 infection or may have been recently infected with the virus. If only the IgM result is positive, the infection may be developing. If only the IgG result is positive, there has been a parvovirus infection in the past, and the person is now protected against this virus. If both tests are negative, the person has never been infected with parvovirus B19 and is therefore not protected.