My Father Is 82 Years Of Age; He Recently Had

My father is 82 years of age; he recently had two surgeries for cranial brain bleeds and had a successful recovery after physical therapy. He is prone to bladder infections and pneumonia. Prior to the onset of these symptoms he is able to walk with a walker, has a good mind and a relatively healthy appetite. Once these conditions take full affect he becomes extremely weak, unable to walk or move himself, has little to no appetite and is seems very confused. He has been tested and has no symptoms of infection, high/low blood pressure or brain bleed. What could cause this drastic change in functional capability?