Dear user, Hilum of the lung is the central area of the lung where the blood vessels and air-tubes go in and come out from that lung. Hilar means near or at the hilum. Pneumonia is infection of a part or whole of the lung. Hilar pneumonia means there is an infective area near to the hilum. Non-infective causes like irritation by an obstructed organic material accidentally inhaled by the child can also produce the same signs on the X ray, and need to be excluded. Joint pains may be an extra problem and is probably not related to the pneumonia that he has now. I think you should definitely visit a specialist ... may be a Pediatrician or a respiratory specialist. - Dr Taher