Pneumonia is due to infections caused primarily by bacteria or viruses and less commonly by fungi and parasites. Although there are more than 100 strains of infectious agents identified, only a few are responsible for the majority of the cases. Mixed infections with both viruses and bacteria may occur in up to 45% of infections in children and 15% of infections in adults. A causative agent may not be isolated in approximately half of cases despite careful testing.

The term "pneumonia" is sometimes more broadly applied to any condition resulting in inflammation of the lungs (caused for example by autoimmune diseases, chemical burns or drug reactions); however, this inflammation is more accurately referred to as pneumonitis.

Conditions and risk factors that predispose to pneumonia include smoking, immunodeficiency, alcoholism, chronic obstructive pulmonary disease, asthma, chronic kidney disease, and liver disease. The use of acid-suppressing medications—such as proton-pump inhibitors or H2 blockers—is associated with an increased risk of pneumonia. The risk is also increased in old age.

Community-acquired pneumonia (CAP) is acquired in the community, outside of health care facilities. Compared with health care–associated pneumonia, it is less likely to involve multidrug-resistant bacteria. Although the latter are no longer rare in CAP, they are still less likely.

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.

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.

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.

Nursing home-acquired pneumonia is an important subgroup of HCAP. Residents of long term care facilities may become infected through their contacts with the healthcare system; as such, the microbes responsible for their pneumonias may be different from those traditionally seen in community-dwelling patients, requiring therapy with different antibiotics. Other groups include patients who are admitted as a day case for regular hemodialysis or intravenous infusion (for example, chemotherapy). Especially in the very old and in demented patients, HCAP is likely to present with atypical symptoms.

The most common causative organisms are (often intracellular living) bacteria:

- "Chlamydophila pneumoniae": Mild form of pneumonia with relatively mild symptoms.

- "Chlamydophila psittaci": Causes psittacosis.

- "Coxiella burnetii": Causes Q fever.

- "Francisella tularensis": Causes tularemia.

- "Legionella pneumophila": Causes a severe form of pneumonia with a relatively high mortality rate, known as legionellosis or Legionnaires' disease.

- "Mycoplasma pneumoniae": Usually occurs in younger age groups and may be associated with neurological and systemic (e.g. rashes) symptoms.

Atypical pneumonia can also have a fungal, protozoan or viral cause.In the past, most organisms were difficult to culture. However, newer techniques aid in the definitive identification of the pathogen, which may lead to more individualized treatment plans.

Since the start of the AIDS epidemic, PCP has been closely associated with AIDS. Because it only occurs in an immunocompromised host, it may be the first clue to a new AIDS diagnosis if the patient has no other reason to be immunocompromised (e.g. taking immunosuppressive drugs for organ transplant). An unusual rise in the number of PCP cases in North America, noticed when physicians began requesting large quantities of the rarely used antibiotic pentamidine, was the first clue to the existence of AIDS in the early 1980s.

Prior to the development of more effective treatments, PCP was a common and rapid cause of death in persons living with AIDS. Much of the incidence of PCP has been reduced by instituting a standard practice of using oral co-trimoxazole (Bactrim / Septra) to prevent the disease in people with CD4 counts less than 200/μL. In populations that do not have access to preventive treatment, PCP continues to be a major cause of death in AIDS.

Lower respiratory infectious disease is the fifth-leading cause of death and the combined leading infectious cause of death, being responsible for 2·74 million deaths worldwide. This is generally similar to estimates in the 2010 Global Burden of Disease study.

This total only accounts for "Streptococcus pneumoniae" and "Haemophilus Influenzae" infections and does not account for atypical or nosocomial causes of lower respiratory disease, therefore underestimating total disease burden.

"Klebsiella" resistant strains have been recorded in USA with a roughly threefold increase in Chicago cases, quarantined individuals in Israel, United Kingdom and parts of Europe, possible ground zero, or location of emergence, is the India-Pakistan border.

A strain known as Carbapenem-Resistant Klebsiella pneumonia (CRKP) was estimated to be involved in 350 cases in Los Angeles county between June and December 2010.

The disease PCP is relatively rare in people with normal immune systems, but common among people with weakened immune systems, such as premature or severely malnourished children, the elderly, and especially persons living with HIV/AIDS (in whom it is most commonly observed). PCP can also develop in patients who are taking immunosuppressive medications. It can occur in patients who have undergone solid organ transplantation or bone marrow transplantation and after surgery. Infections with "Pneumocystis" pneumonia are also common in infants with hyper IgM syndrome, an X-linked or autosomal recessive trait.

The causative organism of PCP is distributed worldwide and "Pneumocystis" pneumonia has been described in all continents except Antarctica. Greater than 75% of children are seropositive by the age of 4, which suggests a high background exposure to the organism. A post-mortem study conducted in Chile of 96 persons who died of unrelated causes (suicide, traffic accidents, and so forth) found that 65 (68%) of them had pneumocystis in their lungs, which suggests that asymptomatic pneumocystis infection is extremely common.

"Pneumocystis jirovecii" was originally described as a rare cause of pneumonia in neonates. It is commonly believed to be a commensal organism (dependent upon its human host for survival). The possibility of person-to-person transmission has recently gained credence, with supporting evidence coming from many different genotyping studies of "Pneumocystis jirovecii" isolates from human lung tissue. For example, in one outbreak of 12 cases among transplant patients in Leiden, it was suggested as likely, but not proven, that human-to-human spread may have occurred.

Acute bronchitis is one of the most common diseases. About 5% of adults are affected and about 6% of children have at least one episode a year. It occurs more often in the winter. More than 10 million people in the United States visit a doctor each year for this condition with about 70% receiving antibiotics which are mostly not needed. There are efforts to decrease the use of antibiotics in acute bronchitis.

In terms of the pathophysiology of Klebsiella pneumonia we see neutrophil myeloperoxidase defense against "K P".Oxidative inactivation of elastase is involved, while LBP helps transfer bacteria cell wall elements to the cells.

As the lungs tend to be vulnerable organs due to their exposure to harmful particles in the air, several things can cause an acute exacerbation of COPD:

- Respiratory infection, being responsible for approximately half of COPD exacerbations. Approximately half of these are due to viral infections and another half appears to be caused by bacterial infections. Common bacterial pathogens of acute exacerbations include "Haemophilus influenzae", "Streptococcus pneumoniae" and "Moraxella catarrhalis". Less common bacterial pathogens include "Chlamydia pneumoniae" and "MRSA". Pathogens seen more frequently in patients with impaired lung function (FEV<35% of predicted) include "Haemophilus parainfluenzae" (after repeated use of antibiotics), "Mycoplasma pneumoniae" and gram-negative, opportunistic pathogens like "Pseudomonas aeruginosa" and "Klebsiella pneumoniae".

- Allergens, e.g., pollens, wood or cigarette smoke, pollution

- Toxins, including a variety of different chemicals

- Air pollution

- Failing to follow a drug therapy program, e.g. improper use of an inhaler

In one-third of all COPD exacerbation cases, the cause cannot be identified.

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.

Gram-negative bacteria are seen less frequently: "Haemophilus influenzae" (), "Klebsiella pneumoniae" (), "Escherichia coli" (), "Pseudomonas aeruginosa" (), "Bordetella pertussis", and "Moraxella catarrhalis" are the most common.

These bacteria often live in the gut and enter the lungs when contents of the gut (such as vomit or faeces) are inhaled.

Eosinophilic pneumonia is a rare disease. Parasitic causes are most common in geographic areas where each parasite is endemic. AEP can occur at any age, even in previously healthy children, though most patients are between 20 and 40 years of age. Men are affected approximately twice as frequently as women. AEP has been associated with smoking. CEP occurs more frequently in women than men and does not appear to be related to smoking. An association with radiation for breast cancer has been described.

Pneumonia is an illness which can result from a variety of causes, including infection with bacteria, viruses, fungi, or parasites. Pneumonia can occur in any animal with lungs, including mammals, birds, and reptiles.

Symptoms associated with pneumonia include fever, fast or difficult breathing, nasal discharge, and decreased activity.

Different animal species have distinct lung anatomy and physiology and are thus

affected by pneumonia differently. Differences in anatomy, immune systems, diet, and behavior also affects the particular microorganisms commonly causing

pneumonia. Diagnostic tools include physical examination, testing of the

sputum, and x-ray investigation. Treatment depends on the cause of pneumonia;

bacterial pneumonia is treated with antibiotics.

"See also:" Pneumonia, Pneumonic.

Atypical bacteria causing pneumonia are "Coxiella burnetii", "Chlamydophila pneumoniae" (), "Mycoplasma pneumoniae" (), and "Legionella pneumophila".

The term "atypical" does not relate to how commonly these organisms cause pneumonia, how well it responds to common antibiotics or how typical the symptoms are; it refers instead to the fact that these organisms have atypical or absent cell wall structures and do not take up Gram stain in the same manner as gram-negative and gram-positive organisms.

Pneumonia caused by "Yersinia pestis" is usually called pneumonic plague.

Most cases of chronic bronchitis are caused by smoking cigarettes or other forms of tobacco. Additionally, chronic inhalation of air pollution or irritating fumes or dust from hazardous exposures in occupations such as coal mining, grain handling, textile manufacturing, livestock farming, and metal moulding may also be a risk factor for the development of chronic bronchitis. Protracted bacterial bronchitis is usually caused by "Streptococcus pneumoniae", "Non-typable Haemophilus influenzae", or "Moraxella catarrhalis".

Sources of such lipids could be either exogenous or endogenous.

Exogenous: from outside the body. For example, inhaled nose drops with an oil base, or accidental inhalation of cosmetic oil. Amiodarone is an anti-arrythmic known to cause this condition. Oil pulling has also been shown to be a cause. At risk populations include the elderly, developmentally delayed or persons with gastroesophageal reflux. Switching to water-soluble alternatives may be helpful in some situations.

Endogenous: from the body itself, for example, when an airway is obstructed, it is often the case that distal to the obstruction, lipid-laden macrophages (foamy macrophages) and giant cells fill the lumen of the disconnected airspace.

The pneumonia presents as a foreign body reaction causing cough, dyspnoea, and often fever. Haemoptysis has also been reported.

Acute exacerbations can be partially prevented. Some infections can be prevented by vaccination against pathogens such as influenza and "Streptococcus pneumoniae". Regular medication use can prevent some COPD exacerbations; long acting beta-adrenoceptor agonists (LABAs), long-acting anticholinergics, inhaled corticosteroids and low-dose theophylline have all been shown to reduce the frequency of COPD exacerbations. Other methods of prevention include:

- Smoking cessation and avoiding dust, passive smoking, and other inhaled irritants

- Yearly influenza and 5-year pneumococcal vaccinations

- Regular exercise, appropriate rest, and healthy nutrition

- Avoiding people currently infected with e.g. cold and influenza

- Maintaining good fluid intake and humidifying the home, in order to help reduce the formation of thick sputum and chest congestion.

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.

Occult pneumonia is a pneumonia that is not observable directly by the eye, but can only be shown indirectly, especially by radiography. Occult pneumonia can be made visible by chest X-rays.

The general symptoms "cough for more than 10 days" and "fever for more than 3 days" can indicate the presence of occult pneumonia, just as a temperature of 39 °C or higher and a high white blood cell count.

Administration of a pneumococcal vaccine decreases the incidence of occult pneumonia, which suggests that "Streptococcus pneumoniae" is a cause of occult pneumonia. Occult pneumonia, however, can also be the result of atypical pneumonia.

Although pneumococcal vaccination lowers the prevalence of occult pneumonia, it does not make radiographic diagnosis superfluous at patients with prolonged fever, cough or leukocytosis.

Etymology: the term is derived from the Latin "occultus" = hidden, secret and "pneumonia" = inflammation of the lungs > Greek: "pneuma" = wind and Indo-European: "pleumon" = floating, swimming.