ATLANTA, Georgia (AP) -- Scientists have made from scratch the Spanish flu virus that killed as many as 50 million people in 1918, the first time an infectious agent behind a historic pandemic has ever been reconstructed. Why did they do it? Researchers say it may help them better understand -- and develop defenses against -- the threat of a future worldwide epidemic from bird flu. Like the 1918 virus, the current avian flu in Southeast Asia occurs naturally in birds. In 1918, the virus mutated, infected people and then spread among them. So far, the current Asian virus has killed at least 65 people but has rarely spread person-to-person. But viruses mutate rapidly and it could soon develop infectious properties like those seen in the 1918 bug, said Dr. Jeffery Taubenberger of the U.S. Armed Forces Institute of Pathology. 以下省略