What Causes Infection At The Surgery Site On The Leg?

Hi, my right leg femur has broken due to trauma & been joined by a plate by a surgery & which is already polio attacked. After 6 months of that surgery it causes infection while I was doing walking practice as per doctor's advice with the help of knee brace tied in leg & walker. Now please tell me, why the infection occurs whether it is due to use of knee brace or any other reason?