The family of Mycobacteriaceae contains the single genus Mycobacterium with over 150 species. They are broadly distributed in various natural environments, and with several notable exceptions, they do not appear in parasitic or disease associations. This genus also includes Mycobacerium tuberculosis, currently responsible for serious bacterial infections of humans in many parts of the world, with millions of deaths annually. Other Mycobacterium species can cause a variety of infections as well; for example, worldwide, there are over quarter million new cases of leprosy, a debilitating disease caused by Mycobacterium leprae. A number of mycobacterial saprophytes have been shown to cause opportunistic infections in immunocompromised individuals, and many are pathogens of animals.