Leprosy is a chronic infectious disease caused by Mycobacterium leprae (M. leprae). It mainly affects the skin, the peripheral nerves and the mucous membranes. It is a disease mainly of human beings, which affects people of all races, all ages and both sexes.

Patients harboring many bacilli in their bodies, the multi bacillary patients, are the main sources of infection. If not treated, they spread the disease in the community and infect others through coughing and sneezing (droplet infection). These infectious patients represent only about 25% of the registered leprosy patients in Tanzania. The other 75% of patients with few leprosy bacilli, the paucibacillary patients are less infectious. Skin contact with leprosy patients is no longer considered to be an important means of transmission.

The different manifestations of leprosy are due to differences in the degree of resistance (immunity) of the human body and not due to different kinds of bacilli.

The majority of people (about 85%) have a strong resistance to M. Leprae that even when infected they do not develop the disease. They are immune. About 75% of children who get infected with leprosy bacilli have such a high resistance that they overcome the disease themselves, without treatment, at very early stage. People who have a fairly high but incomplete immunity to leprosy bacilli will develop paucibacillary leprosy.

There are only very few people in the community (5-10%) whose immunity to M. Leprae is naturally very low. When somebody from this group of people is infected by M. Leprae, the bacilli may multiply freely and attain large numbers causing multi-bacillary leprosy.