Bronchoalveolar lavage (BAL) is a medical procedure in which a bronchoscope is passed through the mouth or nose into the lungs and fluid is squirted into a small part of the lung and then collected for examination. BAL is typically performed to diagnose lung disease. In particular, BAL is commonly used to diagnose infections in people with immune system problems,pneumonia in people on ventilators, some types of lung cancer, and scarring of the lung (interstitial lung disease). BAL is the most common manner to sample the components of the epithelial lining fluid (ELF) and to determine the protein composition of the pulmonary airways, and it is often used in immunological research as a means of sampling cells or pathogen levels in the lung. Examples of these include T-cell populations and influenza viral levels.