A surgical site infection is an infection that occurs after surgery in the part of the body where the surgery took place. Surgical site infections can sometimes be superficial infections involving the skin only. Other surgical site infections are more severe and can involve tissues under the skin, organs, or implanted material. These infections are caused by germs. The most common of these include the bacteria Staphylococcus, Streptococcus, and Gram-negative bacilli.