My 22 Year Old Son Had A Heart Transplant At

My 22 year old son had a Heart Transplant at 3 months old. He has had only one small episode of rejection 1 week after transplant. He has been a poster child! He was diagnosed in July after his Heart Cath with transplant coronary artery disease. He has felt fine....no symptoms at all. His regular checkup yesterday he had a chest xray it showed haziness on the left lung mid section. He has had no symptoms of anything. What could be the cause?