Surgical site infections are caused by bacteria that get in through incisions made during surgery. They threaten the lives of millions of patients each year and contribute to the spread of antibiotic resistance. In low- and middle-income countries, 11% of patients who undergo surgery are infected in the process. In Africa, up to 20% of women who have a caesarean section contract a wound infection, compromising their own health and their ability to care for their babies. But surgical site infections are not just a problem for poor countries. In the United States, they contribute to patients spending more than 400 000 extra days in hospital at a cost of an additional US$ 10 billion per year. No international evidence-based guidelines had previously been available before WHO launched its global guidelines on the prevention of surgical site infection on 3 November 2016, and there are inconsistencies in the interpretation of evidence and recommendations in existing national guidelines. These new WHO guidelines are valid for any country and suitable to local adaptations, and take account of the strength of available scientific evidence, the cost and resource implications, and patient values and preferences.