Leprosy is an ancient disease that has been the cause of great morbidity and mortality for centuries. A 4000-year-old skeleton with evidence of lepromatous leprosy was found in India, and DNA from the causative agent, Mycobacterium leprae, has been isolated from a Byzantine skeleton from Israel dated to 300–600 AD. Mycobacterium leprae is an unculturable, obligate intracellular, gram-positive, acid-fast bacillus. It multiplies very slowly in the host and grows best at 33°C (91.4°F), which accounts for its predilection for cooler parts of the body such as the skin, testis, anterior segment of the eye, mucous membranes of nasal passages, and ear lobes and extremities.
Leprosy appears to have originated in a single clone in East Africa and then spread to Asia, the Middle East, Europe, and by way of the slave trade into West Africa. Phylogenetic studies using medieval European and modern bacterial DNA suggest a European origin of leprosy in the Americas as well as a paleogeographic relationship between Europe and the Middle East. The disease is now endemic in a number of regions, mainly in Asia, Africa, South America, and the Pacific. It is especially prevalent in India and Brazil. India alone accounts for more than 50% of the global leprosy burden of disease. Isolated pockets of disease are found in many parts of the world, and as a consequence of international travel, affected individuals may be encountered in any location. In the United States, infected patients may be found in any state, but most are in California, Hawaii, Florida, Texas, and Louisiana. Most cases encountered in the United States are seen in immigrants born in endemic regions.