U.S. regulators Friday approved the first treatment for smallpox — a deadly disease that was wiped out four decades ago — in case the virus is used in a terror attack. Smallpox, which is highly contagious, was eradicated worldwide by 1980 after a huge vaccination campaign. But people born since then haven’t been vaccinated, and small samples of the smallpox virus were saved for research purposes, leaving the possibility it could be used as a biological weapon. Maker SIGA Technologies of New York has already delivered 2 million treatments that will be stockpiled by the government, which partially paid for...