Toxoplasmosis is caused by infection with Toxoplasma gondii, an obligate intracellular parasite. The infection produces a wide range of clinical syndromes in humans, land and sea mammals, and various bird species. In most immunocompetent individuals, primary or chronic (latent) T gondii infection is asymptomatic. A small percentage of these patients eventually develop chorioretinitis, lymphadenitis, or, rarely, myocarditis and polymyo-sitis. However, certain individuals are at high risk for severe or life-threatening toxoplasmosis. Individuals at risk for toxoplasmosis include fetuses, newborns, and immunologically impaired patients. Congenital toxoplasmosis is usually a subclinical infection. Among immunodefi cient individuals, toxoplasmosis most often occurs in those with defects of T-cell mediated immunity, such as those with hematologic malignancies, bone marrow and solid organ transplants, or AIDS.

Parvovirus B19 (B19V) is a single-stranded DNA virus of the family Parvoviridae and genus Erythrovirus. Human parvovirus B19 was shown to be the etiologic agent of erythema infectiosum in hematologically normal persons. Erythema infectiosum was originally named Fifth disease because it was the fi fth of 6 classic exanthematous diseases of childhood to be described. Later, cases of nonimmune hydrops fetalis were reported when infection in a woman occurred during pregnancy. Patients who are immunocompromised (eg, receiving chemotherapy or immunosuppressive drugs or have immune defects [congenital and acquired]) may develop chronic parvovirus B19 infection that results in chronic anemia. Pure red cell aplasia (PRAC) persists until the virus is cleared and should be distinguished from the transient anemia described above. Chronic parvovirus B19 infection in transplant recipients has been linked to anemia, other hematologic abnormalities, myocarditis, and pneumonitis.

The name rubella is derived from a Latin term meaning Little red. Rubella is generally a benign communicable exanthematous disease and it is caused by Rubella virus, which is a member of the Rubivirus genus of the family Togaviridae. Nearly one half of individuals infected with this virus are asymptomatic. Infection in younger children is characterized by mild constitutional symptoms, rash, and suboccipital adenopathy; conversely, in older children, adolescents, and adults, rubella may be complicated by arthralgia, arthritis, and thrombocytopenic purpura. Rare cases of rubella encephalitis have also been described in children. The major complication of rubella is its teratogenic effects when pregnant women contract the disease, especially in the early weeks of gestation. The virus can be transmitted to the fetus through the placenta and is capable of causing serious congenital defects, abortions, and stillbirths.