In immunointact individuals, infection by the ubiquitous protozoan parasite Toxoplasma gondii is common, but clinical disease is rare; however, fetal and immunocompromised populations are at risk for clinical toxoplasmosis. T. gondii organisms persist as quiescent tissue cysts in various tissues of the body with the possibility of tissue cysts reactivating to actively multiplying parasites if there is a decline in the infected individual's immune system. In more recent years, there has been an increase in toxoplasmosis due to a steadily increasing immunocompromised population. T. gondii infections are controlled principally by the cellular immune system. Thus, individuals with defective cell-mediated immunity cannot control a T. gondii infection and if they have been infected previously, reactivation of a previous infection may occur. Congenital toxoplasmosis can cause severe complications in the fetuses of women who are infected with T. gondii during pregnancy. Toxoplasmosis can be serious in individuals with malignancies or AIDS. Since transplant recipients are immunosuppressed by drug treatment, they too are at risk for toxoplasmosis if they receive an organ from an infected donor. Vaccines against T. gondii suitable for human use have not been developed. No drug is available that can eliminate the encysted stage of the parasite; thus, infected individuals are always at risk for reactivation of the parasite if there is a failure of their immune system. More emphasis should be placed on the elimination of T. gondii by development of drugs which can eliminate the cyst stage in tissues and on development of vaccines suitable for protecting humans against infection or reactivation.

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