In medical isolation in South Texas, 100 miles or so from Mexico's border, is a man who embodies one of U.S. health officials' greatest worries: He is the first person to cross and be held in detention while infected with one of the most severe types of drug-resistant tuberculosis known today.

His three-month odyssey through 13 countries—from his homeland of Nepal through South Asia, Brazil, Mexico, and finally into Texas—shows the way in which dangerous new strains of the disease can migrate across the world unchecked.

Tuberculosis, an ancient, fatal airborne disease, has been treatable for decades with a cocktail of drugs. However, shoddy medical practices world-wide have enabled the bacteria to mutate and, in some cases, become all but untreatable. In recent months The Wall Street Journal has exposed widening TB drug resistance in hot spots like India, and shown that the U.S. is surprisingly unprepared for the growing global problem. Most U.S. cases of drug-resistant TB occur in people who were born abroad, according to the Centers for Disease Control and Prevention.

The Nepalese man detained at the U.S. border carries a particularly deadly strain—XDR, "extensively drug-resistant" TB. His TB is resistant to at least eight of the 15 or so standard drugs, according to a U.S. government description of the case reviewed by the Journal. His XDR strain has been seen only once before in the U.S., in another patient of Nepalese origin, according to the government description.

The Nepalese patient was taken into custody by the U.S. Border Patrol in late November as he tried to cross the border illegally near McAllen, Texas, according to Department of Homeland Security officials. The government declined to name him.