Surgical Site Infection (SSI): This is often the most common cause of post-operative fever, occurring due to bacterial contamination of the surgical site during or after surgery. The risk factors include the type of surgery, patient's overall health, and perioperative care practices.

Pneumonia: Post-operative pneumonia can occur due to aspiration, immobilization, or respiratory complications. It's a common cause of fever, especially in patients who have undergone abdominal or thoracic surgery.
Urinary Tract Infection (UTI): UTIs are common in post-operative patients, especially those with urinary catheters. Bacteria can colonize the catheter and cause infection.
Deep Vein Thrombosis (DVT): While not typically the first diagnosis that comes to mind for fever, DVT can cause a low-grade fever and is a significant concern in post-operative patients due to immobilization.
Atelectasis: This condition, involving the collapse or closure of a lung resulting in reduced or absent gas exchange, can cause fever. It's common after surgeries that affect breathing or require prolonged bed rest.

Sepsis: Although not the most common cause, sepsis is a life-threatening condition that can arise from any infection, including SSIs, UTIs, or pneumonia. Early recognition and treatment are crucial.
Meningitis: In neurosurgical patients, meningitis is a critical diagnosis to consider, as it can be devastating if not promptly treated.
Intra-abdominal Abscess: Especially after abdominal surgeries, an intra-abdominal abscess can be a source of fever and requires early detection and drainage to prevent severe outcomes.
Endocarditis: In patients with pre-existing heart conditions or those who have undergone cardiac surgery, endocarditis is a potentially lethal condition that must be considered in the differential diagnosis of post-operative fever.

Adrenal Insufficiency: This condition can cause fever among other symptoms and is particularly relevant in patients who have undergone adrenal or pituitary surgery, or those on long-term corticosteroid therapy.
Drug Fever: Certain medications can cause fever without an identifiable source of infection. This is more common with certain antibiotics and anti-inflammatory drugs.
Thrombophlebitis: Inflammation of veins due to a blood clot can cause fever and is a consideration in patients with central lines or those who are immobile.
Pancreatitis: Post-operative pancreatitis can occur, especially after surgeries in the vicinity of the pancreas, and may present with fever among other symptoms.