A relatively rare type of multiple drug resistant tuberculosis (MDR-TB). Extensively drug resistant tuberculosis (XDR-TB) occurs when a Mycobacterium tuberculosis strain becomes resistant to drugs used to treat TB, including the two most effective first-line antibiotics (isoniazid and rifampin) and most of the second-line drugs. XDR-TB progresses more rapidly and is more severe in people coinfected with HIV than in people infected with XDR-TB alone.