It's nature's stealth killer. It's not always the medical examiner's prime suspect. And the deadly toll it exacts often becomes clear only well after it has left the scene. Stifling heat has already claimed 17 lives in Maryland this summer, as well as one in the District and nine in Virginia. Those numbers are likely to grow, experts said, because the hot weather's casualty figures are generally counted days and weeks after a heat wave ends. High temperatures claim more lives in the United States than tornadoes, hurricanes, floods and lightning combined -- about 700 a year, according to official estimates. Almost all are preventable. Better understanding can help prevent more deaths, some officials say, by encouraging people to take measures such as drinking fluids and seeking relief in an air-conditioned building, even if for just a few hours a day.