Leprosy exhibits quite a wide range of clinical features in different people and this is now thought to be due to differences in the body's immune response to the infection. Most people have an effective response which completely prevents the disease, while others have only a moderate response which allows the disease to appear, but limits it to only a few skin patches. In these patients, the number of leprosy bacilli in the body is quite small (less than a million) and they don't show up on the skin smear test, which is negative; the disease is classified as paucibacillary (meaning 'few bacilli'). A very small minority of people, on the other hand, have such a weak immune response to the leprosy bacillus that it can multiply almost without any check and spread to almost all parts of the skin and the peripheral nerves. In these patients the skin smear is positive and the disease is classified as multibacillary (meaning 'many bacilli'). For the purposes of treatment, all patients are put into one category or the other (either paucibacillary - PB - or multibacillary - MB), but this is somewhat arbitrary. The disease should be seen as a more or less continuous spectrum from high to low immunity. It should be pointed out that people who get multibacillary leprosy have no other immune deficiencies and have no particular susceptibility to any other disease.

The straightforward classification of leprosy into two treatment groups (PB/MB) is described by WHO. The older, but more detailed, classification of leprosy is known as the Ridley/Jopling classification.