Lung disease refers to several types of diseases or disorders that prevent the lungs from functioning properly. Lung disease can affect respiratory function, or the ability to breathe, and pulmonary function, which is how well lungs work.There are many different lung diseases, some of which are caused by bacterial, viral, or fungal infections. Other lung diseases are associated with environmental factors, including asthma, mesothelioma, and lung cancer.Chronic lower respiratory diseases is a set of conditions that includes chronic obstructive pulmonary disease (COPD), emphysema, and chronic bronchitis. Lung cancer is a disease caused by the abnormal growth of cells. Though most lung cancer starts in the lungs, some cases start in other parts of the body and spread to the lungs. The two main types of lung cancer—small cell and non-small cell—grow and spread in different ways, and each type may be treated differently.Cigarette smoking is the overall leading cause of lung cancer. Breathing secondhand smoke can also increase a person’s chance of developing the disease.