Leprosy is caused by Mycobacterium leprae, a slow-growing, acid-fast bacillus that primarily affects the skin and peripheral nerves. This bacterium has a unique predilection for cooler areas of the body, leading to characteristic skin lesions and nerve damage. The hallmark of leprosy is nerve involvement causing sensory loss and muscle weakness, which can result in deformities if untreated. Diagnosis relies on clinical features and confirmation by skin smear or biopsy demonstrating acid-fast bacilli. Treatment requires prolonged multidrug therapy to prevent resistance and complications.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 20th Edition.