(Respiratory Diseases(श्वसन रोग) , or lung disease, is a medical term that encompasses pathological conditions affecting the organs and tissues that make gas exchange difficult in air-breathing animals. They include conditions of the respiratory tract including the trachea, bronchi, bronchioles, alveoli, pleurae, pleural cavity, and the nerves and muscles of respiration. Respiratory diseases range from mild and self-limiting, such as the common cold, to life-threatening diseases such as bacterial pneumonia, pulmonary embolism, acute asthma and lung cancer. The study of respiratory disease is known as pulmonology. A doctor who specializes in respiratory disease is known as a pulmonologist, a chest medicine specialist, a respiratory medicine specialist, a respirologist or a thoracic medicine specialist. Respiratory diseases can be classified in many different ways, including by the organ or tissue involved, by the type and pattern of associated signs and symptoms, or by the cause of the disease.