Respiratory structures such as the airways, alveoli, and pleural membranes may all be affected by various disease processes. These respiratory diseases include infections such as pneumonia, obstructive disorders that obstruct airflow into and out of the lungs such as asthma, bronchitis, and emphysema, restrictive disorders that limit normal expansion of the lungs such as pneumothorax, atelectasis, respiratory distress syndrome, and cystic fibrosis, cancers, and exposure to inhaled particles that alter pulmonary function. Respiratory infections can occur in the upper respiratory tract, the lower respiratory tract, or both. Organisms capable of infecting respiratory structures include bacteria and viruses.