Leprosy or Hansen’s disease is a chronic granulomatous infection of the skin, peripheral nerves and upper respiratory tract mucosa caused by Mycobacterium leprae, an acid-fast bacillus. Leprosy is an old insidious and neglected disease characterized by tissue damage and demyelinating lesions in peripheral nerves. Leprosy, which was once seen in Europe and Asia, is now endemic in 105 countries, mostly in developing and poor countries with warm temperate tropical climates. Host immunity against M. leprae appears in different clinical patterns, from tuberculoid leprosy with strong production of Th1-type cytokines to lepromatous disease characterized by high levels of Th2-type cytokines and a suboptimal proinflammatory response. In the diagnosis of leprosy, clinical findings, examination of biopsies from the active edge of the lesions by histopathological, bacteriological and molecular methods, and lepromine skin test are used. The aim of this review is to raise awareness to commemorate the 150th anniversary of Dr. Gerhard-Henrik Armauer Hansen’s discovery of M. leprae and review the current information on the pathogenesis and microbiological diagnosis of leprosy, the forgotten and neglected disease, which is still seen in 136 countries of the world.