Africa. The two main groups of human patients at risk for L. infantum infection are young children, human immunodeficiency virus (HIV)-positive patients and other immunocompromised patients. The domestic dog is considered the main reservoir for L. infantum infection. Infection among populations of wild mammals such as black rats, hares, foxes and jackals has been reported in the Mediterranean basin and South America and may also play a role in the epidemiology of L. infantum infection in these regions. This chapter focuses on L. infantum infection in dogs and cats.