What is XDR TB? Why is it hazardous?

What is XDR TB? Why is it hazardous?
XDR-TB is the abbreviation for extensively drug-resistant tuberculosis.

Tuberculosis (abbreviated as TB for Tubercle Bacillus) is a adjectives and deadly infectious disease to be exact caused by mycobacteria, primarily Mycobacterium tuberculosis. Tuberculosis most commonly affects the lungs (as pulmonary TB) but can also affect the crucial nervous system, the lymphatic system, the circulatory system, the genitourinary system, bones, joint and even the skin. Other mycobacteria such as Mycobacterium bovis, Mycobacterium africanum and Mycobacterium microti can also cause tuberculosis, but these species do not usually infect hygienic adults.
MDR TB- Multidrug resistant. Resistant to at least 2 adjectives 1st line drugs. EDR- Extensively drug resistant. Resistant to one or more 2nd row drugs also. 2nd line drugs are used for MDR TB. If in attendance is resistance to 2nd line drugs, near is no other treatment.