Skin lesion. Light micrograph of a section through a tuberculosis skin lesion. Tuberculosis is an infectious disease caused by the bacterium Mycobacterium tuberculosis. Primary infection almost always occurs in the lungs, from where it may spread. There are several forms of cutaneous tuberculosis. If infection extends into the skin from underlying tissues it is known as scrofuloderma. Nodular lesions on the face are called lupus vulgaris. Most common is the formation of tuberculids due to hypersensitivity of the skin to antigens. Lesions affecting the skin are rare and indicate advanced disease, occurring several years after initial infection.