Normal surgical masks and N95 masks appear equivalent with respect to preventing respiratory infections.

Pneumonia occurs in a variety of situations and treatment must vary according to the situation. It is classified as either community or hospital acquired depending on where the patient contracted the infection. It is life-threatening in the elderly or those who are immunocompromised. The most common treatment is antibiotics and these vary in their adverse effects and their effectiveness. Pneumonia is also the leading cause of death in children less than five years of age in low income countries. The most common cause of pneumonia is pneumococcal bacteria, "Streptococcus pneumoniae" accounts for 2/3 of bacteremic pneumonias. This is a dangerous type of lung infection with a mortality rate of around 25%.

For optimal management of a pneumonia patient, the following must be assessed: pneumonia severity (including treatment location, e.g., home, hospital or intensive care), identification of causative organism, analgesia of chest pain, the need for supplemental oxygen, physiotherapy, hydration, bronchodilators and possible complications of emphysema or lung abscess.

The lower respiratory tract consists of the trachea (wind pipe), bronchial tubes, the bronchioles, and the lungs.

Lower respiratory tract infections are generally more serious than upper respiratory infections. LRIs are the leading cause of death among all infectious diseases. The two most common LRIs are bronchitis and pneumonia. Influenza affects both the upper and lower respiratory tracts, but more dangerous strains such as the highly pernicious H5N1 tend to bind to receptors deep in the lungs.

Pulmonary aspiration of acidic material (such as stomach acid) may produce an immediate primary injury caused by the chemical reaction of acid with lung parenchyma, and a later secondary injury as a result of the subsequent inflammatory response.

Lower respiratory tract infections place a considerable strain on the health budget and are generally more serious than upper respiratory infections.

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.

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,

Several studies found that healthcare-associated pneumonia is the second most common type of pneumonia, occurring less commonly than community-acquired pneumonia but more frequently than hospital-acquired pneumonia and ventilator-associated pneumonia. In a recent observational study, the rates for CAP, HCAP and HAP were 60%, 25% and 15% respectively. Patients with HCAP are older and more commonly have simultaneous health problems (such as previous stroke, heart failure and diabetes).

The number of residents in long term care facilities is expected to rise dramatically over the next 30 years. These older adults are known to develop pneumonia 10 times more than their community-dwelling peers, and hospital admittance rates are 30 times higher.

Among the factors contributing to contracting HAP are mechanical ventilation (ventilator-associated pneumonia), old age, decreased filtration of inspired air, intrinsic respiratory, neurologic, or other disease states that result in respiratory tract obstruction, trauma, (abdominal) surgery, medications, diminished lung volumes, or decreased clearance of secretions may diminish the defenses of the lung. Also, poor hand-washing and inadequate disinfection of respiratory devices cause cross-infection and are important factors.

When comparing the bacterial-caused atypical pneumonias with these caused by real viruses (excluding bacteria that were wrongly considered as viruses), the term "atypical pneumonia" almost always implies a bacterial cause and is contrasted with viral pneumonia.

Known viral causes of atypical pneumonia include respiratory syncytial virus (RSV), influenza A and B, parainfluenza, adenovirus, severe acute respiratory syndrome (SARS)

and measles.

Bronchomalacia can best be described as a birth defect of the bronchus in the respiratory tract. Congenital malacia of the large airways is one of the few causes of irreversible airways obstruction in children, with symptoms varying from recurrent wheeze and recurrent lower airways infections to severe dyspnea and respiratory insufficiency. It may also be acquired later in life due to chronic or recurring inflammation resulting from infection or other airway disease.

Mycoplasma is found more often in younger than in older people.

Older people are more often infected by Legionella.

Obstructive lung disease is a category of respiratory disease characterized by airway obstruction. Many obstructive diseases of the lung result from narrowing (obstruction) of the smaller bronchi and larger bronchioles, often because of excessive contraction of the smooth muscle itself. It is generally characterized by inflamed and easily collapsible airways, obstruction to airflow, problems exhaling and frequent medical clinic visits and hospitalizations. Types of obstructive lung disease include; asthma, bronchiectasis, bronchitis and chronic obstructive pulmonary disease (COPD). Although COPD shares similar characteristics with all other obstructive lung diseases, such as the signs of coughing and wheezing, they are distinct conditions in terms of disease onset, frequency of symptoms and reversibility of airway obstruction. Cystic fibrosis is also sometimes included in obstructive pulmonary disease.

There is low or very-low quality evidence that probiotics may be better than placebo in preventing acute URTIs. Vaccination against influenza viruses, adenoviruses, measles, rubella, "Streptococcus pneumoniae", "Haemophilus influenzae", diphtheria, "Bacillus anthracis", and "Bordetella pertussis" may prevent them from infecting the URT or reduce the severity of the infection.

Diagnosis of obstructive disease requires several factors depending on the exact disease being diagnosed. However one commonalty between them is an FEV1/FVC ratio less than 0.7, i.e. the inability to exhale 70% of their breath within one second.

Following is an overview of the main obstructive lung diseases. "Chronic obstructive pulmonary disease" is mainly a combination of chronic bronchitis and emphysema, but may be more or less overlapping with all conditions.

According to a Cochrane review, single oral dose of nasal decongestant in the common cold is modestly effective for the short term relief of congestion in adults; however, "there is insufficient data on the use of decongestants in children." Therefore, decongestants are not recommended for use in children under 12 years of age with the common cold. Oral decongestants are also contraindicated in patients with hypertension, coronary artery disease, and history of bleeding strokes.

Bronchomalacia is a term for weak cartilage in the walls of the bronchial tubes, often occurring in children under six months. Bronchomalacia means 'floppiness' of some part of the bronchi. Patients present with noisy breathing and/or wheezing. There is collapse of a main stem bronchus on exhalation. If the trachea is also involved the term tracheobronchomalacia (TBM) is used. If only the upper airway the trachea is involved it is called tracheomalacia (TM). There are two types of bronchomalacia. Primary bronchomalacia is due to a deficiency in the cartilaginous rings. Secondary bronchomalacia may occur by extrinsic compression from an enlarged vessel, a vascular ring or a bronchogenic cyst. Though uncommon, idiopathic (of unknown cause) tracheobronchomalacia has been described in older adults.

Tracheitis is an inflammation of the trachea.

Although the trachea is usually considered part of the lower respiratory tract, in ICD-10 tracheitis is classified under "acute upper respiratory infections".

Clinically, the most serious and immediate complication is acute respiratory distress syndrome (ARDS), which usually occurs within 24 h. Those with significant lower airway involvement may develop bacterial infection. Importantly, victims suffering body surface burn and smoke inhalation are the most susceptible. Thermal injury combined with inhalation injury compromises pulmonary function, producing microvascular hyperpermeability that leads to a significant increase in lung lymph flow and pulmonary edema. The terrorist attack on the World Trade Center on September 11, 2001 left many people with impaired lung function. A study of firefighters and EMS workers enrolled in the FDNY WTC Medical Monitoring and Treatment Program, whose lung function was tested prior to 9/11, documented a steep decline in lung function in the first year after 9/11. A new study that includes a thousand additional workers shows that the declines have persisted over time. Prior to 9/11, 3% of firefighters had below-normal lung function, one year after 9/11 nearly 19% did, and six years later it stabilized at 13%. Ten to 14 days after acute exposure to some agents (e.g. ammonia, nitrogen oxides, sulfur dioxide, mercury), some patients develop bronchiolitis obliterans progressing to ARDS. Bronchiolitis obliterans with organized pneumonia can ensue when granulation tissue accumulates in the terminal airways and alveolar ducts during the body's reparative process. A minority of these patients develop late pulmonary fibrosis. Also at enhanced risk are persons with co-morbidities. Several studies report that both aged persons and smokers are especially vulnerable to the adverse effects of inhalation injury.

Bacterial tracheitis is a bacterial infection of the trachea and is capable of producing airway obstruction.

One of the most common causes is "Staphylococcus aureus" and often follows a recent viral upper respiratory infection. Bacterial tracheitis is a rare complication of influenza infection. It is the most serious in young children, possibly because of the relatively small size of the trachea that gets easily blocked by swelling. The most frequent sign is the rapid development of stridor. It is occasionally confused with croup.

If it is inflamed, a condition known as tracheitis can occur. In this condition there can be inflammation of the linings of the trachea. A condition called tracheo-bronchitis can be caused, when the mucous membrane of the trachea and bronchi swell. A collapsed trachea is formed as a result of defect in the cartilage, that makes the cartilage unable to support the trachea and results in dry hacking cough. In this condition there can be inflammation of the linings of the trachea. If the connective nerve tissues in the trachea degenerate it causes tracheomalacia. Infections to the trachea can cause tracheomegaly.

Post-viral cough can be resistant to treatment. Post-viral cough usually goes away on its own; however, cough suppressants containing codeine may be prescribed. A study has claimed theobromine in dark chocolate is more effective.

One possible cause for post-viral cough is that the receptors that are responsible for stimulating the cough during the respiratory tract infection are up-regulated by respiratory tract infection and continue to stimulate even after the virus has disappeared.

Test articles passing muster in vitro can be evaluated in a number of in vivo models (usually in mice) of ALI including chlorine inhalation, intratracheal instillation of bleomycin and in transforming growth factor β1 (TGF β1) overexpressing transgenic mice exposed to high dose doxycycline. Acute exposure to high concentrations of chlorine gas induces pathological and functional changes in the lungs of rodents. Histological changes consist of epithelial necrosis and detachment, increase in the area of smooth muscle, epithelial regeneration and mucous cell hyperplasia. Most of these abnormalities resolve with time. Functional changes (increased RL and/or bronchial responsiveness to inhaled methacholine) last for mean intervals of 3 and 7 days after exposure, but can persist up to 30 and 90 days, respectively. The functional changes are related to the overall abnormal airway epithelial damage and there is a significant correlation between RL and bronchoalveolar lavage ( BAL) neutrophilia. Bleomycin is an antineoplastic antibiotic drug isolated in 1966 from the actinomycete Streptomyces verticillus. Bleomycin forms a complex with oxygen and metals such as Fe2+, leading to the production of oxygen radicals, DNA breaks, and ultimately cell death. Doxycycline driven overexpression of TGF β1 in the lungs of transgenic mice result in a time-dependent inflammatory response characterized by massive infiltration of F4/80+ monocytic/macrophage-like cells and a wave of apoptotic pulmonary cell death. Mice that survive this initial onslaught go on to demonstrate an increase in lung collagen content, and decreased lung compliance. A large animal model of ALI is the ovine model of body surface burn + heated smoke inhalation. It has been established that combined burn and smoke inhalation injury impairs hypoxic pulmonary vasoconstriction (HPV), the vasoconstrictive response to hypoxia, thereby mismatching ventilation with perfusion. Gas exchange is affected by increases in the dispersion of both alveolar ventilation and cardiac output because bronchial and vascular functions are altered by injury-related factors, such as the effects of inflammatory mediators on airway and vascular smooth muscle tone. As a rule of thumb, all these models are characterized by high mortality, inflammation of the airways and pulmonary parenchyma, edema and flooding of the alveolar spaces by a proteinaceous exudate, sloughing of the airway and pulmonary epithelium, scarring and transition to airway and pulmonary remodeling.

There is currently a limited amount of information available on the incidence and prevalence of VCD, and the various rates reported in the literature are most likely an underestimate. Although VCD is thought to be rare overall, its prevalence among the population at large is not known.

However, numerous studies have been conducted on its incidence and prevalence among patients presenting with asthma and exertional dyspnea. A VCD incidence rate of 2% has been reported among patients whose primary complaint was either asthma or dyspnea; the same incidence rate has also been reported among patients with acute asthma exacerbation. Meanwhile, much higher VCD incidence rates have also been reported in asthmatic populations, ranging from 14% in children with refractory asthma to 40% in adults with the same complaint. It has also been reported that the VCD incidence rate is as high as 27% in non-asthmatic teenagers and young adults.

Data on the prevalence of VCD is also limited. An overall prevalence of 2.5% has been reported in patients presenting with asthma. Among adults with asthma considered "difficult to control", 10% were found to have VCD while 30% were found to have both VCD and asthma. Among children with severe asthma, a VCD prevalence rate of 14% has been reported. However, higher rates have also been reported; among one group of schoolchildren thought to suffer from exercise-induced asthma, it was found that 26.9% actually had VCD and not asthma. Among intercollegiate athletes with exercise-induced asthma, the VCD rate has been estimated at 3%.

In patients presenting with symptoms of dyspnea, prevalence rates ranging from 2.8% to 22% have been reported in various studies. It has been reported that two to three times more females than males suffer from VCD. VCD is especially common in females who suffer from psychological problems. There is an increased risk associated with being young and female. Among patients suffering from VCD, 71% are over the age of 18. In addition, 73% of those with VCD have a previous psychiatric diagnosis. VCD has also been reported in newborns with gastroesophageal reflux disorder (GERD).