Leprosy exhibits a wide range of different clinical features in different people. This is thought to be due to differences in the body's immune response to the infection.
Most people have an effective immune response to leprosy which completely prevents the disease from developing, 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 bacilli load does not 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 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 assigned into one of these two categories but this is an arbitrary process. The disease should be seen as a more or less continuous spectrum from high to low immunity. It is of note that people who contract multibacillary leprosy often 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.