Leishmania is a parasite transmitted by the bite of a female sand fly of the genus Lutzomyia. The insect injects humans or other animals with promastigotes, the infective stage of the parasite. Once injected into the skin, the promastigote is consumed by immune system cells such as macrophages and other mononuclear phagocytic cells. Within these cells, the promastigote transforms into the tissue stage of the parasite, known as an amastigote, which multiplies inside the cell by simple division, moving on to infect other phagocytic mononuclear cells. Various factors of the parasite and host determine which form of the disease appears in the host. The insects become infected by sucking infected cells of the host during a blood meal. In the insect´s gut, the cells rupture, releasing amastigotes, which are transformed back into promastigotes. They multiply and develop in the gut. After several days, depending on the species, the parasites migrate to the mouthparts of the insect, where they are ready again to be transmitted to a host during the next blood meal.