CD4 and severe combined immunodeficiency: In particular, systemic risk factors include immunocompromised patients such as individuals suffering from severe combined immunodeficiency (SCID), chronic granulomatous disease (CGD), haematological malignancies (in particular chronic lymphoid leukaemia), and graft-versus-host disease (GVHD) or patients affected by HIV with a CD4+ T cell count below 50/μL.