Pulmonary fibrosis (literally “scarring of the lungs”) is a respiratory disease in which scars are formed in the lung tissues, leading to serious breathing problems. Scar formation, the accumulation of excess fibrous connective tissue (the process called fibrosis), leads to thickening of the walls, and causes reduced oxygen supply in the blood. As a consequence patients suffer from perpetual shortness of breath. There is no medical cure for healing the scars and damages in the lung due to pulmonary fibrosis. Scientists, however, are researching the use of stem cells to regenerate lung tissue.