Tuberculosis - The Threat Returns A deadly new strain of tuberculosis called extremely drug-resistant tuberculosis (XDR-TB) may soon pose a global health threat. Mycobacterium tuberculosis, the bacterium that causes tuberculosis, is transmitted by coughing and can usually be treated with antibiotics. XDR-TB, which is untreatable with all first- and most second-line medications, sprang into existence when patients would improperly take antibiotics for the disease, allowing the bacteria to develop resistance and multiply. Immunocompromised persons, such as HIV patients, are especially susceptible to XDR-TB and to tuberculosis in general. Each year, tuberculosis infects 8.8 million and kills as many as 1.6 million people worldwide.