A survey of 100 healthy adults reveals that 80% have IgG antibodies to Toxoplasma. Which one of the following statements would help to explain this finding?

Serologic tests, such as the Sabin-Feldman dye test and indirect immunofluorescence, have shown that a high percentage of the world's population has been infected with Toxoplasma gondii. In adults, clinical toxoplasmosis usually presents as a benign syndrome resembling infectious mononucleosis. However, fetal infections are often severe and associated with hydrocephalus, chorioretinitis, convulsions, and death. Acute toxoplasmosis is best diagnosed by an IgM capture assay. In most patients, specific IgM antibody disappears within 3 to 6 months.