Co-infection with HIV and any of the Leishmania species that cause VL can result in failure of the CD4 lymphocyte count to rise above 200 cells/mm3 despite sustained, effective suppression of HIV infection, and this failure of CD4 recovery then commonly prevents immune mediated eradication of VL despite aggressive, prolonged anti-Leishmania treatment. The gene discussed is CD4; the disease is HIV infectious disease.