M. leprae belongs to a large group of intracellular bacterial pathogens widely distributed in nature. Members of the family Mycobacteriaceae can infect mammalian and avian hosts, and, as exemplified by bovine tuberculosis and avian tuberculosis, a pathogen dominant in one host species can successfully infect an altogether different host. Thus humans have suffered from bovine tuberculosis transmitted through contaminated cow's milk and from atypical mycobacterial infections in the form of "scrofula" or infected lymphatic glands. But among these, only leprosy and tuberculosis can be successfully transmitted from one human to another, although M. leprae is the only one of these genera that cannot be transmitted naturally to species other than humans.

This feature of leprosy made difficult the search for a suitable experimental animal for research, and only during the mid-1960s did medical scientists succeed in transmitting the infection to armadillos of the American Southwest. In what period of human history M. leprae appeared, and from what other mycobacterial species it evolved are a mystery. Paleopathological evidence confirms the existence of true leprosy in the ancient eastern Mediterranean basin. It was probably African in origin and steadily disseminated among human communities from Pleistocene times onward (Grmek 1983).

The organism usually enters a new human host through respiratory passages or the skin. Because the period of incubation (i.e., the interval between infection and the manifestation of symptoms) is so long, the actual process of the microorganism's growth and dissemination through the body is not well understood. Commonly, early symptoms of disease appear 3 to 5 years after infection, but clinical evidence of infection may appear in as little as 6 months, or as long as 20 years post infection.

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