Tuberculosis Tuberculosis (TB) is an infectious disease caused by a germ (bacterium) called Mycobacterium tuberculosis. This germ primarily affects the lungs and may infect In the United States, the number of TB cases steadily decreased until 1986 when an increase was noted; TB has continued to rise since. Today, ten million individuals are infected in the U.S., as evidenced by positive skin tests, with approximately 26,000 new cases of active disease each year. The increase in TB cases is related to HIV/AIDS, homelessness, drug abuse and immigration of persons with active infections. How is TB Contracted? TB is a contagious or inf