Joseph Lister's most important contribution to medical science was to establish principles of hygiene and cleanliness in surgery. Prior to Lister's introduction of sterile surgery, such precautions were commonly neglected and patients often died from "ward fever," a postoperative infection.
Lister became a Professor of Surgery after moving to Glasgow in 1860. During this time, he became interested in the work of Pasteur and began to experiment with dressing wounds with carbolic acid, or phenol. His experimentation greatly reduced the frequency of infection.
Lister lowered this frequency even further by introducing hand-washing, the sterilization of surgical instruments and a carbolic spray in operating theaters.
Dr. Lister's contributions were so profound that he is remembered as the "father of antiseptic surgery." In fact, Listerine mouthwash, while not invented by Lister, was named in honor of him.