In the early years of the AIDS pandemic, most diagnosed cases of histoplasmosis were identified in the traditional endemic region in the U.S. A prospective study of HIV-infected patients in Missouri showed that CD4 lymphocyte counts <300/μL significantly increased risk for histoplasmosis, and that risk increased sharply in the subgroup with CD counts <150/μL [11]. This evidence concerns the gene CD4 and histoplasmosis.