In an effort to reduce the number of primates subjected to experimentation, we used biological material generated during this study to provide evidence that recovery from acute Lassa fever is associated with an LASV-specific CD4+ and CD8+ T-cell response, whereas fatal Lassa fever is characterized by the expansion of immunosuppressive myeloid cells, alterations of the stromal network of lymph nodes (LNs), and suppression of the virus-specific T-cell response. The gene discussed is CD4; the disease is Lassa fever.