Leishmaniasis is among the more common and serious of the protozoan infections of humans in tropical countries. These are intracellular parasites which infect cells of the immune system of mammals, such as the phagocytes. The parasite reproduces asexually, kills its host cell and reinfects numerous adjacent cells. One result is ulcers form internally or externally near the site of infection. More seriously, the continual infection and destruction of host defense cells causes the blood-forming organs, the spleen and bone marrow, to produce more phagocytes at the expense of erythrocyte production. Severe anemia, emaciation, and death may occur.