What did Joseph Lister discover to kill germs?

He used carbolic acid to sterilize surgical equipment, and also insisted on the use of clean gowns, closed Windows in the operating theatre, and sterilizing the room and its surroundings prior to surgery.

Post surgery, he maintained a similar vigilance on the need for sterile environments. It was mistakenly believed that risks occurred during surgeries and less so afterwards. He helped changed this view.