From a Washington Post article on General Petraeus:

Twice, accidents almost ended his career, or even his life. In 1991, as a battalion commander at Fort Campbell, Ky., he was shot in the chest with an M-16 rifle when a soldier tripped during a training exercise. Rushed into surgery at Vanderbilt University Medical Center in Nashville, he underwent five hours of surgery by Bill Frist, who a decade later became Senate majority leader.