Lung macrophage probing a lung cell for invading pathogens. Dee Breger/Science Photo Library; Coloured x ray of the chest of a patient with pulmonary fibrosis or interstitial lung disease. PHT/Science Photo Library; Bacterial lung infection. Computer artwork of rod-shaped bacteria on the surface of the alveoli of the lungs, causing a lower respiratory tract infection. David Mack/Science Photo Library; and Landmark Design.