In apparently immunocompetent patients, Leishmania infantum provokes a spectrum of disease, ranging from simple skin lesion to severe visceral leishmaniasis, that is determined mainly by the protozoan genotype. In HIV-positive individuals, leishmanial infection results almost exclusively in visceral disease. In this review, Luigi Gradoni and Marina Gramiccia discuss the role of the intrinsic virulence of L. infantum strains and the immune condition of the host and focus on recently described mechanisms of immunological control of leishmanial infection.