Leishmaniasis, leesh muh NY uh sihs, is a disease that causes skin sores in human beings and other animals. It is caused by infection with single-celled parasites of the genus Leishmania. The parasites are transmitted through the bite of various species of sand flies. Leishmaniasis occurs mainly in tropical countries, particularly in Africa, the Middle East, southern Asia, and South America. Each year, about 2 million people are infected with leishmaniasis worldwide.