Diseases of the respiratory system can be categorized into four main groups: infectious, inflammatory, environmental, and cancer. Infectious comprise of upper and respiratory tract bacterial, fungal, and viral infections. Inflammatory are more specifically, reactive airway conditions, while environmental are more related to chemical exposure. Cancerous lesions of the lungs can be caused by environmental factors and many environmental conditions are terminal cancers, yet the distinction is that all environmental diseases are preventable while not all cancers are.
Infectious Respiratory Diseases
The Centers for Disease Control states that the most common infectious condition of the respiratory tract is the common cold. This is a viral syndrome produced by the rhinovirus which largely affects the upper airway. Other viral infections that can causes respiratory diseases include the respiratory syncytial virus which can be deadly to infants and the elderly, while healthy people can recover in 1 to 2 weeks without permanent damage. It is the most common cause of pneumonia and bronchiolitis in children less than 1 year of age. Influenza is another virus that attacks the lung parenchyma and can result in severe respiratory distress and even death in susceptible individuals. Bacterial infections typically produce lobular pneumonias, with the most common bacteria being Mycoplasma species. Mycoplasma pneumonia is often times termed as walking pneumonia, atypical pneumonia, or community-acquired pneumonia. While mycoplasma affects young adults, streptococcus pneumonia more likely affects the elderly and children, along with Legionella species which results in Legionnaires' disease. Lastly, fungal infections produce a ball-shaped appearance on x-rays as described by the American Thoracic Society. Fungal infections occur more readily in immunocompromised individuals whom have inhaled aerosolized fungi most often where the soil is disturbed such as after earthquakes or during farming. Aspergillus is the leading cause of fungal respiratory infections worldwide.
Inflammatory Lung Diseases
Asthma and reactive airway diseases are the most common inflammatory lung conditions in children, according to the Nationwide Children's Hospital. Reactive airway diseases have triggers, such as animal dander or mold, that cause the airways to become inflamed and narrow, thus producing a characteristic wheezing. The muscles around the bronchi tighten in spasms and the mucosal membranes swell making breathing more difficult and if not treated immediately can result in death. Children are more susceptible to these conditions due to their smaller airways. In addition to asthma, adults can experience other inflammatory respiratory diseases such as sarcoidosis, which is an immune system mediated disease. The American Lung Association describes sarcoidosis as granulomatous disease that is characterized by small patches of inflamed cells and results in pulmonary fibrosis. This condition affects more than the lungs as opposed to asthma, which is an inflammation of mainly the bronchial tubes.
Environmental Respiratory Diseases
Environmental respiratory diseases include a wide variety of conditions, ranging from chronic obstructive pulmonary disease or COPD to asbestos lung. First off COPD which the American Lung Association describes as typically caused from a long-standing history of smoking, is considered an inflammatory lung disease, but it requires a history of smoking in most situations. In rare instances, emphysema, a subtype of COPD results from a genetical disorder known as alpha1-antitrypsin deficiency. COPD is the most common of the environmental diseases, being the third leading cause of death in the United States. It is a condition where the air exchange in the lungs has decreased exponentially, making it very difficult to breathe out carbon dioxide. Other environmental diseases as described by Occupational Medicine Agius.com are related to work-exposure of particular chemicals, such the pneumoconiosis diseases. These include silicosis caused by inhalation of silicon dioxide found in quartz, coal worker's lung from inhalation of coal dust, and asbestosis. Chemical pneumonitis is the last of this group which is produced by an inhalation of a variety of chemicals including ammonia, mustard gas, pesticides, sulphur dioxide, ozone, and chlorine to name a few.
The last group of respiratory diseases is lung cancer. According to the Cancer Research UK center, there are several types of cancer that develop first in pulmonary tissue. Other cancers can metastasize to the lung such as liver and breast cancer, but they are not considered true respiratory diseases. The type of lung cancer that most people think of because of its strong association with smoking is small cell lung cancer, but it makes up only twelve percent of pulmonary cancers. This is a fast spreading cancer that requires chemotherapy rather than surgery. The most common lung cancer overall is not associated with smoking and is typically discovered on diagnostic imaging at the outer edges of the lungs. Another type of cancer that is associated with smoking is squamous cell carcinoma and appears most often in the center of a lung wrapped around a main airway. Other types of lung cancer are either large cell carcinoma, which gathers in severely fast growing tumors or unknown types, otherwise known as undifferentiated non small cell carcinoma.