Atypical lymphocytosis is most likely to be found in which one of the following diseases?

Atypical lymphocytes are the hematologic hallmark of infectious mononucleosis with 90% or more of the circulating lymphocytes being atypical in some cases. These abnormal lymphocytes are not pathognomonic for infectious mononucleosis. They are also seen in other diseases, including cytomegalovirus infection, viral hepatitis, toxoplasmosis, rubella, mumps, and roseola.