June 1 (Reuters) - Health authorities in the United States and Europe are tracing about 100 airline passengers and crew who may have been in contact with a man infected with a hard-to-treat form of tuberculosis called XDR TB.

Here are some facts about tuberculosis and the XDR strain:

-- About one-third of the world's population is infected with tuberculosis, although the infection does not cause active disease in most people. TB kills more adults than any other single infectious agent, according to the World Health Organization, with 1.6 million deaths a year.

-- Like other bacteria, tuberculosis can develop what is known as resistance to antibiotics. Any microbe mutates, and when patients take antibiotics, a few evolve ways to survive the drug's action.

-- Multi-drug resistant TB is now widespread and WHO recommends a treatment approach called Directly Observed Treatment, in which patients are watched to make sure they take all their drugs on time and properly.

-- A strain called extensively drug resistant TB survives almost all drugs used to treat TB, including the two best first-line drugs: isoniazid and rifampin. XDR TB is also resistant to the best second-line medications: fluoroquinolones and at least one of three injectable drugs.

-- WHO has confirmed 269 cases of XDR in 35 countries, with 85 percent of the patients expected to die.

-- In the United States, 49 cases of XDR TB have been reported between 1993 and 2006. Long-term treatment with strong antibiotics can cure patients, but the costs exceed $250,000 per patient.

-- The AIDS virus pandemic has helped TB become more widespread, as TB more easily infects and sickens people with suppressed immune systems.

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