In a surgical patient, the causes of non surgical infection are all, except

Surgical site infections (SSIs)  are the third most frequently reported nosocomial infection, first one being pneumonia and the second,  catheter-related  (including urinary  tract infection). SSI within 24 hours is caused by clostridia and streptococci. SSI  after 48  hours  (5  days)  is  caused  by  gram -ve  and other  bacteria. Clostridium difficile diarrhea occurrence is due to administration of various antibiotics. ​Reference:  Manipal Manual of Surgery 4th ed page no 26, 28