Respiratory disease is a common and significant cause of illness and death around the world. In the US, approximately 1 billion "common colds" occur each year. A study found that in 2010, there were approximately 6.8 million emergency department visits for respiratory disorders in the U.S. for patients under the age of 18. In 2012, respiratory conditions were the most frequent reasons for hospital stays among children.

In the UK, approximately 1 in 7 individuals are affected by some form of chronic lung disease, most commonly chronic obstructive pulmonary disease, which includes asthma, chronic bronchitis and emphysema.

Respiratory diseases (including lung cancer) are responsible for over 10% of hospitalizations and over 16% of deaths in Canada.

In 2011, respiratory disease with ventilator support accounted for 93.3% of ICU utilization in the United States.

Pulmonary diseases may also impact newborns, such as pulmonary hyperplasia, pulmonary interstitial emphysema (usually preterm births), and infant respiratory distress syndrome,

Asthma is a respiratory disease that can begin or worsen due to exposure at work and is characterized by episodic narrowing of the respiratory tract. Occupational asthma has a variety of causes, including sensitization to a specific substance, causing an allergic response; or a reaction to an irritant that is inhaled in the workplace. Exposure to various substances can also worsen pre-existing asthma. People who work in isocyanate manufacturing, who use latex gloves, or who work in an indoor office environment are at higher risk for occupational asthma than the average US worker. Approximately 2 million people in the US have occupational asthma.

Health care professionals are at risk of occupational influenza exposure; during a pandemic influenza, anyone in a close environment is at risk, including those in an office environment.

The annual incidence of ARDS is 13–23 people per 100,000 in the general population. Its incidence in the mechanically ventilated population in intensive care units is much higher. According to Brun-Buisson "et al" (2004), there is a prevalence of acute lung injury (ALI) of 16.1% percent in ventilated patients admitted for more than 4 hours.

Worldwide, severe sepsis is the most common trigger causing ARDS. Other triggers include mechanical ventilation, sepsis, pneumonia, Gilchrist's disease, drowning, circulatory shock, aspiration, traumaespecially pulmonary contusionmajor surgery, massive blood transfusions, smoke inhalation, drug reaction or overdose, fat emboli and reperfusion pulmonary edema after lung transplantation or pulmonary embolectomy. Pneumonia and sepsis are the most common triggers, and pneumonia is present in up to 60% of patients and may be either causes or complications of ARDS. Alcohol excess appears to increase the risk of ARDS. Diabetes was originally thought to decrease the risk of ARDS, but this has shown to be due to an increase in the risk of pulmonary edema. Elevated abdominal pressure of any cause is also probably a risk factor for the development of ARDS, particularly during mechanical ventilation.

The death rate varies from 25–40% in centers using up-to-date ventilatory strategies and up to 58% in all centers.

Bronchiolitis typically affects infants and children younger than two years, principally during the fall and winter . Bronchiolitis hospitalization has a peak incidence between two and six months of age and remains a significant cause of respiratory disease during the first two years of life. It is a leading cause of hospitalization in infants and young children.

Since ARDS is an extremely serious condition which requires invasive forms of therapy it is not without risk. Complications to be considered include the following:

- Pulmonary: barotrauma (volutrauma), pulmonary embolism (PE), pulmonary fibrosis, ventilator-associated pneumonia (VAP)

- Gastrointestinal: bleeding (ulcer), dysmotility, pneumoperitoneum, bacterial translocation

- Cardiac: abnormal heart rhythms, myocardial dysfunction

- Kidney: acute kidney failure, positive fluid balance

- Mechanical: vascular injury, pneumothorax (by placing pulmonary artery catheter), tracheal injury/stenosis (result of intubation and/or irritation by endotracheal tube

- Nutritional: malnutrition (catabolic state), electrolyte deficiency.

The rate of BPD varies among institutions, which may reflect neonatal risk factors, care practices (e.g., target levels for acceptable oxygen saturation), and differences in the clinical definitions of BPD.

The term usually refers to acute viral bronchiolitis, a common disease in infancy. This is most commonly caused by respiratory syncytial virus (RSV, also known as human pneumovirus). Other viruses which may cause this illness include metapneumovirus, influenza, parainfluenza, coronavirus, adenovirus, and rhinovirus.

Children born prematurely (less than 35 weeks), with a low birth weight or who have from congenital heart disease may have higher rates of bronchiolitis and are more likely to require hospital admission. There is evidence that breastfeeding provides some protection against bronchiolitis.

Pneumonia is a form of acute respiratory infection that affects the lung parenchyma and oxygenation. When a patient with pneumonia is an alcoholic, the mortality rate exceeds by 50% if they are placed into intensive care (ICU). According to Kershaw, C 2008 page 1, "[a]s of 2001, pneumonia was the sixth most common cause of death in the United States". Alcoholics are at an increased risk for infection with tissue-damaging gram-negative pathogens or for the spread of bacteria in the blood.

DPB has its highest prevalence among the Japanese, at 11 per 100,000 population. Korean, Chinese, and Thai individuals with the disease have been reported as well. A genetic predisposition among East Asians is suggested. The disease is more common in males, with the male to female ratio at 1.4–2:1 (or about 5 men to 3 women). The average onset of the disease is around age 40, and two-thirds of those affected are non-smokers, although smoking is not believed to be a cause. The presence of HLA-Bw54 increases the risk of diffuse panbronchiolitis 13.3-fold.

In Europe and the Americas, a relatively small number of DPB cases have been reported in Asian immigrants and residents, as well as in individuals of non-Asian ancestry. Misdiagnosis has occurred in the West owing to less recognition of the disease than in Asian countries. Relative to the large number of Asians living in the west, the small number of them thought to be affected by DPB suggests non-genetic factors may play some role in its cause. This rarity seen in Western Asians may also be partly associated with misdiagnosis.

Risk factors for pulmonary aspiration include conditions which depress the level of consciousness (such as traumatic brain injury, alcohol intoxication, drug overdose, and general anesthesia). A decreased gag reflex, upper esophageal sphincter and lower esophageal sphincter tone, gastroesophageal reflux, full stomach, as well as obesity, stroke, and pregnancy can all increase the risk of aspiration in the semiconscious. Tracheal intubation or presence of a gastric tube (for example, a feeding tube) may also increase the risk.

Pulmonary aspiration of particulate matter may result in acute airway obstruction which may rapidly lead to death from arterial hypoxemia.

The mechanisms of alcoholic lung disease are:

- Metabolism of alcohol reduces glutathione anti-oxidant levels in the lungs.

- Oxidation damage to the cells impairs the ability of the lungs to remove fluid.

- Oxidative damage to cells reduces immune response.

- Oxidative damage to cells results in a reduced ability to recover from injury.

These chemical changes compound the negative mechanical and microbiological effects of alcoholism on the respiratory system. These include impaired gag reflex and cilia function and greater likelihood of colonies of pneumococcal bacteria in the upper respiratory system.

Although lung damage from concurrent smoking and drug use is often indistinguishable from alcoholic lung disease, there is support for considering alcoholic lung disease as an independent syndrome. Over the last decade, evidence from epistemological studies show that alcohol abuse alone can increase by as much as fourfold the risk for acute respiratory distress syndrome.

Shortness of breath is the primary reason 3.5% of people present to the emergency department in the United States. Of these individuals, approximately 51% are admitted to the hospital and 13% are dead within a year. Some studies have suggested that up to 27% of people suffer from dyspnea, while in dying patients 75% will experience it. Acute shortness of breath is the most common reason people requiring palliative care visit an emergency department.

Congestive heart failure frequently presents with shortness of breath with exertion, orthopnea, and paroxysmal nocturnal dyspnea. It affects between 1–2% of the general United States population and occurs in 10% of those over 65 years old. Risk factors for acute decompensation include high dietary salt intake, medication noncompliance, cardiac ischemia, dysrhythmias, renal failure, pulmonary emboli, hypertension, and infections. Treatment efforts are directed towards decreasing lung congestion.

There are limited national and international studies into the burden of ABPA, made more difficult by a non-standardized diagnostic criteria. Estimates of between 0.5–3.5% have been made for ABPA burden in asthma, and 1–17.7% in CF. Five national cohorts, detecting ABPA prevalence in asthma (based on GINA estimates), were used in a recent meta-analysis to produce an estimate of the global burden of ABPA complicating asthma. From 193 million asthma sufferers worldwide, ABPA prevalence in asthma is estimated between the extremes of 1.35–6.77 million sufferers, using 0.7–3.5% attrition rates. A compromise at 2.5% attrition has also been proposed, placing global burden at around 4.8 million people affected. The Eastern Mediterranean region had the lowest estimated prevalence, with a predicted case burden of 351,000; collectively, the Americas had the highest predicted burden at 1,461,000 cases. These are likely underestimates of total prevalence, given the exclusion of CF patients and children from the study, as well as diagnostic testing being limited in less developed regions.

Untreated DPB leads to bronchiectasis, respiratory failure, and death. A journal report from 1983 indicated that untreated DPB had a five-year survival rate of 62.1%, while the 10-year survival rate was 33.2%. With erythromycin treatment, individuals with DPB now have a much longer life expectancy due to better management of symptoms, delay of progression, and prevention of associated infections like "P. aeruginosa". The 10-year survival rate for treated DPB is about 90%. In DPB cases where treatment has resulted in significant improvement, which sometimes happens after about two years, treatment has been allowed to end for a while. However, individuals allowed to stop treatment during this time are closely monitored. As DPB has been proven to recur, erythromycin therapy must be promptly resumed once disease symptoms begin to reappear. In spite of the improved prognosis when treated, DPB currently has no known cure.

Giving the mother glucocorticoids speeds the production of surfactant. For very premature deliveries, a glucocorticoid is given without testing the fetal lung maturity. The American College of Obstetricians and Gynecologists (ACOG), Royal College of Medicine, and other major organizations have recommended antenatal glucocorticoid treatment for women at risk for preterm delivery prior to 34 weeks of gestation. Multiple courses of glucocorticoid administration, compared with a single course, does not seem to increase or decrease the risk of death or neurodevelopmental disorders of the child.

In pregnancies of greater than 30 weeks, the fetal lung maturity may be tested by sampling the amount of surfactant in the amniotic fluid by amniocentesis, wherein a needle is inserted through the mother's abdomen and uterus. Several tests are available that correlate with the production of surfactant. These include the lecithin-sphingomyelin ratio ("L/S ratio"), the presence of phosphatidylglycerol (PG), and more recently, the surfactant/albumin (S/A) ratio. For the L/S ratio, if the result is less than 2:1, the fetal lungs may be surfactant deficient. The presence of PG usually indicates fetal lung maturity. For the S/A ratio, the result is given as mg of surfactant per gm of protein. An S/A ratio 55 indicates mature surfactant production(correlates with an L/S ratio of 2.2 or greater).

There is evidence to show that steroids given to babies less than 8 days old can prevent bronchopulmonary dysplasia. However, the risks of treatment may outweigh the benefits.

It is unclear if starting steroids more than 7 days after birth is harmful or beneficial. It is thus recommended that they only be used in those who cannot be taken off of a ventilator.

Most cases of COVID-19 are not severe enough to require mechanical ventilation or alternatives, but a percentage of cases are. The type of respiratory support for individuals with COVID-19 related respiratory failure is being actively studied for people in hospital, with some evidence that intubation can be avoided with a high flow nasal cannula or bi-level positive airway pressure. Whether either of these two leads to the same benefit for people who are critically ill is not known. Some doctors prefer staying with invasive mechanical ventilation when available because this technique limits the spread of aerosol particles compared to a high flow nasal cannula.

Severe cases are most common in older adults (those older than 60 years, and especially those older than 80 years). Many developed countries do not have enough hospital beds per capita, which limits a health system's capacity to handle a sudden spike in the number of COVID-19 cases severe enough to require hospitalisation. This limited capacity is a significant driver behind calls to "flatten the curve"—to lower the speed at which new cases occur and thus keep the number of persons sick at any one time lower. One study in China found 5% were admitted to intensive care units, 2.3% needed mechanical support of ventilation, and 1.4% died. In China, approximately 30% of people in hospital with COVID-19 are eventually admitted to ICU.

Acute respiratory distress syndrome (ARDS) has some similarities to IRDS. Transient tachypnea of the newborn presents with respiratory distress syndrome in the preterm newborn.

Main article: Acute respiratory distress syndrome

Mechanical ventilation becomes more complex as acute respiratory distress syndrome (ARDS) develops in COVID-19 and oxygenation becomes increasingly difficult. Ventilators capable of pressure control modes and high PEEP are needed to maximise oxygen delivery while minimising the risk of ventilator-associated lung injury and pneumothorax. High PEEP may not be available on older ventilators.

Type 1 respiratory failure is defined as a low level of oxygen in the blood (hypoxemia) without an increased level of carbon dioxide in the blood (hypercapnia), and indeed the PCO may be normal or low. It is typically caused by a ventilation/perfusion (V/Q) mismatch; the volume of air flowing in and out of the lungs is not matched with the flow of blood to the lungs. The basic defect in type 1 respiratory failure is failure of oxygenation characterized by:

This type of respiratory failure is caused by conditions that affect oxygenation such as:

- Low ambient oxygen (e.g. at high altitude)

- Ventilation-perfusion mismatch (parts of the lung receive oxygen but not enough blood to absorb it, e.g. pulmonary embolism)

- Alveolar hypoventilation (decreased minute volume due to reduced respiratory muscle activity, e.g. in acute neuromuscular disease); this form can also cause type 2 respiratory failure if severe

- Diffusion problem (oxygen cannot enter the capillaries due to parenchymal disease, e.g. in pneumonia or ARDS)

- Shunt (oxygenated blood mixes with non-oxygenated blood from the venous system, e.g. right to left shunt)

CAP is common worldwide, and a major cause of death in all age groups. In children, most deaths (over two million a year) occur in newborn period. According to a World Health Organization estimate, one in three newborn deaths are from pneumonia. Mortality decreases with age until late adulthood, with the elderly at risk for CAP and its associated mortality.

More CAP cases occur during the winter than at other times of the year. CAP is more common in males than females, and more common in black people than Caucasians. Patients with underlying illnesses (such as Alzheimer's disease, cystic fibrosis, COPD, tobacco smoking, alcoholism or immune-system problems) have an increased risk of developing pneumonia.