During his time in Philadelphia, Levy conducted research on Epstein-Barr virus (EBV) with Gertrude and Werner Henle at Children's Hospital and on B lymphocyte biology at the Wistar Institute with Dr. Vittorio Defendi. While studying tumor viruses, particularly retroviruses, at NIH, he discovered xenotropic viruses
Xenotropic viruses replicate or reproduce in cells other than those of the host species. These studies provided support for the germline transmission of endogenous retroviruses and the use of retroviruses in human gene therapy. The search for xenotropic viruses in human cells led to characterization of retrovirus-like particles in placentas. His work at the NIH on the FBJ (Finkel-Biskis-Jinkins)
osteosarcoma virus provided the background for the subsequent discovery of the fos/jun oncogene. == AIDS research ==