Leprosy is caused by a slow-growing bacillus, Mycobacterium leprae. It is transmitted via droplets from the nose and mouth of untreated patients with severe disease, but is not highly infectious. If left untreated, the disease can cause nerve damage, leading to muscle weakness and atrophy, and permanent disabilities. Leprosy can be easily treated with a 6–12-month course of multidrug therapy. The treatment is highly effective, and has few side-effects and low relapse rates; there is no known drug resistance.