A private philanthropy has announced a $600 million initiative to fund risky but potentially lifesaving medical research by 56 of America's top scientists. The Howard Hughes Medical Institute, it says, is expanding its flagship investigators program to nurture new class of scientists. By endowing scientists' research over many years, the institute hopes they will make major discoveries in a variety of fields, including genetics and biology. The scientists, chosen from more than 1,000 applicants, say they want to answer such questions as how climate affects the spread of cholera, malaria, and other infectious diseases and whether doctors can apply the engineering behind the building of airplanes and computers to the human immune system.