A new test that can detect multiple-drug-resistant tuberculosis in two days instead of the standard two to three months promises to help significantly improve treatment and prevent the spread of the airborne infection, the World Health Organization said on Monday. Multiple-drug-resistant TB, or MDR-TB, is a growing public health problem in the world. Five percent of new TB cases are resistant to first-line drugs. That is 450,000 of the nine million new TB cases that are detected each year, the W.H.O. says. In the United States, the prevalence of drug-resistant tuberculosis among foreign-born TB patients has been about 1.5 percent,...