William Stewart Halsted, M.D. was an American surgeon who emphasized strict aseptic technique during surgical procedures, was an early champion of newly discovered anesthetics, and introduced several new operations, including the radical mastectomy for breast cancer. Along with William Osler, Howard Atwood Kelly and William H. Welch, Halsted was one of the "Big Four" founding professors at the Johns Hopkins Hospital. His operating room at Johns Hopkins Hospital is in Ward G, and was described as a small room where medical discoveries and miracles took place. According to an intern who once worked in Halsted's operating room, Halsted had unique techniques, operated on the patients with great confidence and often had perfect results which astonished the interns.

Asepsis is the state of being free from disease-causing micro-organisms. There are two categories of asepsis: medical and surgical. The modern day notion of asepsis is derived from the older antiseptic techniques, a shift initiated by different individuals in the 19th century who introduced practices such as the sterilizing of surgical tools and the wearing of surgical gloves during operations. The goal of asepsis is to eliminate infection, not to achieve sterility. Ideally, a surgical field is sterile, meaning it is free of all biological contaminants, not just those that can cause disease, putrefaction, or fermentation. Even in an aseptic state, a condition of sterile inflammation may develop. The term often refers to those practices used to promote or induce asepsis in an operative field of surgery or medicine to prevent infection.