XDR-TB

Extensively drug-resistant tuberculosis (XDR-TB) is defined as tuberculosis that is resistant to rifampicin and isoniazid (resistance to these first line anti-TB drugs defines Multi-drug-resistant tuberculosis), as well as to any member of the quinolone family and at least one of the following second-line TB treatments: kanamycin, capreomycin, or amikacin.

The old case definition of XDR-TB is MDR-TB that is also resistant to three or more of the six classes of second-line drugs. This definition should no longer be used.

The principles of treatment for MDR-TB and for XDR-TB are the same. The main difference is that XDR-TB is associated with a much higher mortality rate than MDR-TB, because of a reduced number of effective treatment options.

The epidemiology of XDR-TB is currently not well studied, but it is believed that XDR-TB does not transmit easily in healthy populations, yet is capable of causing epidemics in populations which are already stricken by HIV and therefore more susceptible to TB infection.

Source:
http://en.wikipedia.org/wiki/XDR-TB