May 31 (Reuters) - Health authorities in the United States and Europe are tracking down about 100 passengers and airline 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 TB transmission on aircraft:

-- The U.S. Centers for Disease Control and Prevention tracked down more than 2,600 people on 191 flights on which seven highly infectious TB patients flew between 1992 and 1994. Only two confirmed infections were found.

-- Both were latent infections, meaning the people never developed tuberculosis disease.

-- Transmission of TB usually requires close and sustained contact. The Mycobacterium tuberculosis bacillus, a type of bacteria, is usually carried in particles of spit or mucus.

-- The World Health Organization says the "quality of the air on board commercial aircraft is high, and under normal conditions cabin air is cleaner than the air in most buildings. On short flights, there is minimal risk of disease transmission."

-- Health experts say TB transmission is only likely on flights of eight hours or more, and then only between people sitting fairly close -- in the same row, two rows ahead and two rows behind.

-- WHO says because millions of people travel by air every year, it is rarely possible to medically assess passengers before they board.

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