The most frequent source of postoperative sepsis is a surgical site infection (SSI) because it directly involves the area of tissue trauma caused by the surgery. SSIs provide a localized environment where bacteria can proliferate due to disruption of normal barriers, presence of necrotic tissue, and foreign materials like sutures or implants. Early identification and management are crucial to prevent systemic spread leading to sepsis. While other infections contribute to postoperative sepsis, the site of surgery remains the primary focus for infection control and prevention.

Reference: Sabiston Textbook of Surgery, 21st Edition.