Phlebotomine proboscides are too short to penetrate protective clothing, so that sandflies must feed on exposed skin. Most infections with Leishmania species lead to self-healing skin ulcers, which leave the patient permanently scarred but immune to reinfection with related species of Leishmania. However, more serious sequelae are noted in some forms of leishmaniasis. A primary skin ulcer does not always precede the onset of the most fatal form of leishmaniasis, kala-azar, in which the Leishmania invade the entire reticuloendothelial system. Kala-azar smoulders for years in endemic foci, to erupt into fulminating epidemics in times of migration, famine, and war.
Leishmanial disease occurs on every continent except Australia and Antarctica.
There are at least four different species causing human disease in the Old World, and at least three in the New World. All Leishmania species can cause skin ulcers, but only some of them can cause more severe disease in immunogenetically predisposed individuals. Yet because not all infected persons in groups at high risk for serious sequelae develop severe disease, other factors are probably involved. So it seemed likely that early immunological events occurring in the skin could critically determine the outcome of infection with Leishmania. The doses used were not high enough to cause sunburn, but were about the amount of UV-B in an hour's exposure to sunlight in the semitropics on a bright day at noon.