Leishmania mexicanum is a trypanomastid protazoan (unicellular eukaryotic) parasite found in Central and South America. It causes the most mild form of leishmaniasis, a flesh-eating disease of the tropics, and is transmitted to its human host by the sand fly.
Infection occurs when a sandfly feeds on the blood of the host, simultaneously injecting the promastigote form of the parasite into the blood. The promastigotes are phagocytized by the macrophages of the immune system, inside of which the parasites become amastigotes and multiply. An uninfected sandfly becomes infected by consuming blood from an infected host, which contains macrophages infected with amastigotes. Inside the midgut of the sandfly, the amatsigotes becomes promastigotes, divide, and migrate to the proboscis, which is used by the vector to feed on blood.
Leishmania mexicanum can cause both a cutaneous and diffuse cutaneous infection. In other words, this parasite can cause a ulcers at the bite cite that do not spread, and eventually heal on their own within the span of a few days to a few months following infection. In some cases, a diffuse cutaneous infection occurs when the amastigote form of the parasite spreads cutaneously, resulting in sores and ulcers all over the host's body. Although these sores generally heal on their own, secondary infections can occur and permanent disfigurement can result.