Introduction

Lung disease refers to disorders that affect the lungs, the organs that allow us to breathe. Breathing problems caused by lung disease may prevent the body from getting enough oxygen. Lung diseases are some of the most common medical conditions in the world. Smoking, infections, and genes cause most lung diseases. Your lungs are part of a complex system, expanding and relaxing thousands of times each day to bring in oxygen and send out carbon dioxide. Lung disease can happen when there are problems in any part of this system.

Experts don't know the causes of all types of lung disease, but they do know the causes of some. These include:

• Smoking: Smoke from cigarettes, cigars, and pipes is the number one cause of lung disease. Don't start smoking, or quit if you already smoke. If you live or work with a smoker, avoid secondhand smoke. Ask smokers to smoke outdoors. Secondhand smoke is especially bad for babies and young children.
• Asbestos: This is a natural mineral fibre that is used in insulation, fireproofing materials, car brakes, and other products. Asbestos can give off small fibres that are too small to be seen and can be inhaled. Asbestos harms lung cells, causing lung scarring and lung cancer.
• Air pollution: Recent studies suggest that some air pollutants like car exhaust may contribute to asthma, COPD, lung cancer, and other lung diseases.

Some diseases that affect the lungs, like the flu, are caused by germs (bacteria, viruses, and fungi).

Early signs of lung disease are easy to overlook. Often, an early sign of lung disease is not having your usual level of energy. The signs and symptoms can differ by type of lung disease. Common signs are:

• Trouble breathing
• Shortness of breath
• Feeling like you're not getting enough air
• Decreased ability to exercise
• A cough that won't go away
• Coughing up blood or mucus
• Pain or discomfort when breathing in or out

Lung Diseases Affecting the Airways

The windpipe (trachea) branches into tubes called bronchi, which in turn become smaller tubes throughout your lungs. Diseases that can affect these airways include:

• Asthma: Your airways are constantly inflamed and may spasm, causing wheezing and shortness of breath. Allergies, infections, or pollution can trigger asthma symptoms.
• Chronic obstructive pulmonary disease (COPD): With this lung condition, you cant exhale the way you usually would, which causes trouble breathing.
• Chronic bronchitis: This form of COPD brings a long-term wet cough.
•  Emphysema: Lung damage allows air to be trapped in your lungs in this form of COPD. Trouble blowing air out is its hallmark.
• Acute bronchitis: This sudden infection of your airways is usually caused by a virus.
• Cystic fibrosis: With this condition, you have trouble clearing mucus out of your bronchi. This leads to repeated lung infections.

Lung Diseases Affecting the Air Sacs (Alveoli)

Your airways branch into tiny tubes (bronchioles) that end in clusters of air sacs called alveoli. These air sacs make up most of your lung tissue. Lung diseases affecting your alveoli include:

• Pneumonia: An infection of your alveoli, usually by bacteria or viruses, including the coronavirus that causes COVID-19.
• Tuberculosis: Pneumonia slowly gets worse, caused by the bacteria Mycobacterium tuberculosis. 
• Emphysema: This happens when the fragile links between alveoli are damaged. Smoking is the usual cause. 
• Pulmonary edema: Fluid leaks out of the small blood vessels of your lung into the air sacs and the area around them. One form is caused by heart failure and back pressure in your lungs' blood vessels. In another form, injury to your lung causes the leak of fluid.
• Lung cancer: It has many forms and may start in any part of your lungs. It most often happens in the main part of your lung, in or near the air sacs.
• Acute respiratory distress syndrome (ARDS): This is a severe, sudden injury to the lungs from a serious illness. COVID-19 is one example. Many people who have ARDS need help to breathe from a machine called a ventilator until their lungs recover.
• Pneumoconiosis: This is a category of conditions caused by inhaling something that injures your lungs. Examples include black lung disease from coal dust and asbestosis from asbestos dust.

Lung Diseases Affecting the Interstitium

The interstitium is the thin, delicate lining between your alveoli. Tiny blood vessels run through the interstitium and let gas transfer between the alveoli and your blood. Various lung diseases affect the interstitium:

• Interstitial lung disease (ILD): This is a group of lung conditions that includes sarcoidosis, idiopathic pulmonary fibrosis and autoimmune disease.
• Pneumonia and pulmonary edema can also affect your interstitium.

Lung Diseases Affecting Blood Vessels

The right side of your heart gets low-oxygen blood from your veins. It pumps blood into your lungs through the pulmonary arteries. These blood vessels can have diseases, as well.

• Pulmonary embolism(PE): A blood clot (usually in a deep leg vein, called deep vein thrombosis) breaks off, travels to the heart, and gets pumped into the lungs. The clot sticks in a pulmonary artery, often causing shortness of breath and low blood oxygen levels.
• Pulmonary hypertension: Many conditions can cause high blood pressure in your pulmonary arteries. This can lead to shortness of breath and chest pain.  

Lung Diseases Affecting the Pleura

The pleura is the thin lining that surrounds your lung and lines the inside of your chest wall. A tiny layer of fluid lets the pleura on your lung's surface slide along the chest wall with each breath. Lung diseases of the pleura include:

• Pleural effusion: Fluid collects in the space between your lung and the chest wall. Pneumonia or heart failure usually causes this. Large pleural effusions can make it hard to breathe and may need to be drained.
• Pneumothorax: Air may get into the space between your chest wall and the lung, collapsing the lung.
• Mesothelioma: This is a rare form of cancer that forms on the pleura. Mesothelioma tends to happen several decades after you come into contact with asbestos.

 Lung Diseases Affecting the Chest Wall

The chest wall also plays an important role in breathing. Muscles connect your ribs to each other, helping your chest expand. The diaphragm descends with each breath, also causing chest expansion. Diseases that affect your chest wall include:

• Obesity hypoventilation syndrome: Extra weight on the chest and belly can make it hard for your chest to expand. This may cause serious breathing problems. 
• Neuromuscular disorders: You might have trouble breathing when the nerves that control your respiratory muscles dont work the way they should. Amyotrophic lateral sclerosis and myasthenia gravis are examples of neuromuscular lung disease.