New or progressive infiltrate on the chest X-ray with one of the following:

- Fever > 37.8 °C (100 °F)

- Purulent sputum

- Leukocytosis > 10,000 cells/μl

In an elderly person, the first sign of hospital-acquired pneumonia may be mental changes or confusion.

Other symptoms may include:

- A cough with greenish or pus-like phlegm (sputum)

- Fever and chills

- General discomfort, uneasiness, or ill feeling (malaise)

- Loss of appetite

- Nausea and vomiting

- Sharp chest pain that gets worse with deep breathing or coughing

- Shortness of breath

- Decreased blood pressure and fast heart rate

Individuals with "Klebsiella" pneumonia tend to cough up a characteristic sputum, as well as having fever, nausea, tachycardia and vomiting. "Klebsiella" pneumonia tends to affect people with underlying conditions, such as alcoholism.

Usually the atypical causes also involve atypical symptoms:

- No response to common antibiotics such as sulfonamide and beta-lactams like penicillin.

- No signs and symptoms of lobar consolidation, meaning that the infection is restricted to small areas, rather than involving a whole lobe. As the disease progresses, however, the look can tend to lobar pneumonia.

- Absence of leukocytosis.

- Extrapulmonary symptoms, related to the causing organism.

- Moderate amount of sputum, or no sputum at all (i.e. non-productive).

- Lack of alveolar exudate.

- Despite general symptoms and problems with the upper respiratory tract (such as high fever, headache, a dry irritating cough followed later by a productive cough with radiographs showing consolidation), there are in general few physical signs. The patient looks better than the symptoms suggest.

Signs and symptoms of PCP include fever, non-productive cough (because sputum is too viscous to become productive), shortness of breath (especially on exertion), weight loss, and night sweats. There is usually not a large amount of sputum with PCP unless the patient has an additional bacterial infection. The fungus can invade other visceral organs (such as the liver, spleen, and kidney), but only in a minority of cases.

Pneumothorax is a well-known complication of PCP. An acute history of chest pain with breathlessness and diminished breath sounds is typical of pneumothorax.

Hospital-acquired pneumonia (HAP) or nosocomial pneumonia refers to any pneumonia contracted by a patient in a hospital at least 48–72 hours after being admitted. It is thus distinguished from community-acquired pneumonia. It is usually caused by a bacterial infection, rather than a virus.

HAP is the second most common nosocomial infection (after urinary tract infections) and accounts for 15–20% of the total. It is the most common cause of death among nosocomial infections and is the primary cause of death in intensive care units.

HAP typically lengthens a hospital stay by 1–2 weeks.

Community-acquired pneumonia (CAP) refers to pneumonia (any of several lung diseases) contracted by a person with little contact with the healthcare system. The chief difference between hospital-acquired pneumonia (HAP) and CAP is that patients with HAP live in long-term care facilities or have recently visited a hospital. CAP is common, affecting people of all ages, and its symptoms occur as a result of oxygen-absorbing areas of the lung (alveoli) filling with fluid. This inhibits lung function, causing dyspnea, fever, chest pains and cough.

CAP, the most common type of pneumonia, is a leading cause of illness and death worldwide. Its causes include bacteria, viruses, fungi and parasites. CAP is diagnosed by assessing symptoms, making a physical examination and on x-ray. Other tests, such as sputum examination, supplement chest x-rays. Patients with CAP sometimes require hospitalization, and it is treated primarily with antibiotics, antipyretics and cough medicine. Some forms of CAP can be prevented by vaccination and by abstaining from tobacco products.

Over 100 microorganisms can cause CAP, with most cases caused by "Streptococcus pneumoniae". Certain groups of people are more susceptible to CAP-causing pathogens; for example, infants, adults with chronic conditions (such as chronic obstructive pulmonary disease), senior citizens, alcoholics and others with compromised immune systems are more likely to develop CAP from "Haemophilus influenzae" or "Pneumocystis carinii". A definitive cause is identified in only half the cases.

The cause of the condition Klebsiella pneumonia is "Klebsiella pneumoniae" which is gram-negative, as well as rod-shaped, glucose-fermenting, facultative anaerobic bacterium.

"Streptococcus pneumoniae" () is the most common bacterial cause of pneumonia in all age groups except newborn infants. "Streptococcus pneumoniae" is a Gram-positive bacterium that often lives in the throat of people who do not have pneumonia.

Other important Gram-positive causes of pneumonia are "Staphylococcus aureus" () and "Bacillus anthracis".

People with infectious pneumonia often have a productive cough, fever accompanied by shaking chills, shortness of breath, sharp or stabbing chest pain during deep breaths, and an increased rate of breathing. In the elderly, confusion may be the most prominent sign.

The typical signs and symptoms in children under five are fever, cough, and fast or difficult breathing. Fever is not very specific, as it occurs in many other common illnesses, may be absent in those with severe disease, malnutrition or in the elderly. In addition, a cough is frequently absent in children less than 2 months old. More severe signs and symptoms in children may include blue-tinged skin, unwillingness to drink, convulsions, ongoing vomiting, extremes of temperature, or a decreased level of consciousness.

Bacterial and viral cases of pneumonia usually present with similar symptoms. Some causes are associated with classic, but non-specific, clinical characteristics. Pneumonia caused by "Legionella" may occur with abdominal pain, diarrhea, or confusion, while pneumonia caused by "Streptococcus pneumoniae" is associated with rusty colored sputum, and pneumonia caused by "Klebsiella" may have bloody sputum often described as "currant jelly". Bloody sputum (known as hemoptysis) may also occur with tuberculosis, Gram-negative pneumonia, and lung abscesses as well as more commonly with acute bronchitis. "Mycoplasma" pneumonia may occur in association with swelling of the lymph nodes in the neck, joint pain, or a middle ear infection. Viral pneumonia presents more commonly with wheezing than does bacterial pneumonia. Pneumonia was historically divided into "typical" and "atypical" based on the belief that the presentation predicted the underlying cause. However, evidence has not supported this distinction, thus it is no longer emphasized.

Bacterial pneumonia is a type of pneumonia caused by bacterial infection.

People who are on mechanical ventilation are often sedated and are rarely able to communicate. As such, many of the typical symptoms of pneumonia will either be absent or unable to be obtained. The most important signs are fever or low body temperature, new purulent sputum, and hypoxemia (decreasing amounts of oxygen in the blood).

Lobar pneumonia usually has an acute progression.

Classically, the disease has four stages:

- Congestion in the first 24 hours: This stage is characterized histologically by vascular engorgement, intra-alveolar fluid, small numbers of neutrophils, often numerous bacteria. Grossly, the lung is heavy and hyperemic

- Red hepatization or consolidation: Vascular congestion persists, with extravasation of red cells into alveolar spaces, along with increased numbers of neutrophils and fibrin. The filling of airspaces by the exudate leads to a gross appearance of solidification, or consolidation, of the alveolar parenchyma. This appearance has been likened to that of the liver, hence the term "hepatization".

- Grey hepatization: Red cells disintegrate, with persistence of the neutrophils and fibrin. The alveoli still appear consolidated, but grossly the color is paler and the cut surface is drier.

- Resolution (complete recovery): The exudate is digested by enzymatic activity, and cleared by macrophages or by cough mechanism. Enzymes produced by neutrophils will liquify exudates, and this will either be coughed up in sputum or be drained via lymph.

Chest radiographs (X-ray photographs) often show a pulmonary infection before physical signs of atypical pneumonia are observable at all.

This is occult pneumonia. In general, occult pneumonia is rather often present in patients with pneumonia and can also be caused by "Streptococcus pneumoniae", as the decrease of occult pneumonia after vaccination of children with a pneumococcal vaccine suggests.

Infiltration commonly begins in the perihilar region (where the bronchus begins) and spreads in a wedge- or fan-shaped fashion toward the periphery of the lung field. The process most often involves the lower lobe, but may affect any lobe or combination of lobes.

Ventilator-associated pneumonia is a type of lung infection that occurs in people who are on mechanical ventilation breathing machines in hospitals. As such, VAP typically affects critically ill persons that are in an intensive care unit (ICU). VAP is a major source of increased illness and death. Persons with VAP have increased lengths of ICU hospitalization and have up to a 20-30% death rate. The diagnosis of VAP varies among hospitals and providers but usually requires a new infiltrate on chest x-ray plus two or more other factors. These factors include temperature of >38 °C or 12 × 10/ml, purulent secretions from the airways in the lung, and/or reduction in gas exchange.

"Pneumocystis" pneumonia (PCP) is a form of pneumonia, caused by the yeast-like fungus "Pneumocystis jirovecii".

"Pneumocystis" pneumonia is not commonly found in the lungs of healthy people, but, being a source of opportunistic infection, it can cause a lung infection in people with a weak immune system. "Pneumocystis" pneumonia is especially seen in people with cancer undergoing chemotherapy, HIV/AIDS, and the use of medications that suppress the immune system.

Pneumonia can cause respiratory failure by triggering acute respiratory distress syndrome (ARDS), which results from a combination of infection and inflammatory response. The lungs quickly fill with fluid and become stiff. This stiffness, combined with severe difficulties extracting oxygen due to the alveolar fluid, may require long periods of mechanical ventilation for survival.

Sepsis is a potential complication of pneumonia but occurs usually in people with poor immunity or hyposplenism. The organisms most commonly involved are "Streptococcus pneumoniae", "Haemophilus influenzae", and "Klebsiella pneumoniae". Other causes of the symptoms should be considered such as a myocardial infarction or a pulmonary embolism.

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.

The clinical presentation of both the adult and pediatric patient with pleural empyema depends upon several factors, including the causative micro-organism. Most cases present themselves in the setting of a pneumonia, although up to one third of patients do not have clinical signs of pneumonia and as many as 25% of cases are associated with trauma (including surgery). Typical symptoms include cough, chest pain, shortness of breath and fever.

Pneumococcal pneumonia is a type of bacterial pneumonia that is specifically caused by Streptococcus pneumoniae. "S. pneumoniae" is also called pneumococcus. It is the most common bacterial pneumonia found in adults. The estimated number of Americans with pneumococcal pneumonia is 900,000 annually, with almost 400,000 cases hospitalized and fatalities accounting for 5-7% of these cases.

The symptoms of pneumococcal pneumonia can occur suddenly, typically presenting as a severe chill, later including a severe fever, cough, shortness of breath, rapid breathing, and chest pains. Other symptoms like nausea, vomiting, headache, fatigue, and muscle aches could also accompany the original symptoms. Sometimes the coughing can produce rusty or blood-streaked sputum. In 25% of cases, a parapneumonic effusion may occur. Chest X-rays will typically show lobar consolidation or patchy infiltrates.

In most cases, once pneumococcal pneumonia has been identified, doctors will prescribe antibiotics. These antibiotic usually help alleviate and eliminate symptoms between 12 and 36 hours after being taken. Despite most antibiotics' effectiveness in treating the disease, sometimes the bacteria can resist the antibiotics, causing symptoms to worsen. Additionally, age and health of the infected patient can contribute to the effectiveness of the antibiotics. A vaccine has also been developed for the prevention of pneumococcal pneumonia, recommended to children under age five as well as adults over the age of 65.

While it has been commonly known that the influenza virus increases one's chances of contracting pneumonia or meningitis caused by the streptococcus pneumonaie bacteria, new medical research in mice indicates that the flu is actually a necessary component for the transmission of the disease. Researcher Dimitri Diavatopoulo from the Radboud University Nijmegen Medical Centre in the Netherlands describes his observations in mice, stating that in these animals, the spread of the bacteria only occurs between animals already infected with the influenza virus, not between those without it. He says that these findings have only been inclusive in mice, however, he believes that the same could be true for humans.

Lobar pneumonia is a form of pneumonia that affects a large and continuous area of the lobe of a lung.

It is one of the two anatomic classifications of pneumonia (the other being bronchopneumonia).

Lower respiratory tract infection (LRTI), while often used as a synonym for pneumonia, can also be applied to other types of infection including lung abscess and acute bronchitis. Symptoms include shortness of breath, weakness, fever, coughing and fatigue.

There are a number of symptoms that are characteristic of lower respiratory tract infections. The two most common are bronchitis and edema. Influenza affects both the upper and lower respiratory tracts.

Antibiotics are the first line treatment for pneumonia; however, they are not effective or indicated for parasitic or viral infections. Acute bronchitis typically resolves on its own with time.

In 2015 there were about 291 million cases. These resulted in 2.74 million deaths down from 3.4 million deaths in 1990. This was 4.8% of all deaths in 2013.

Bronchitis describes the swelling or inflammation of the bronchial tubes. Additionally, bronchitis is described as either acute or chronic depending on its presentation and is also further described by the causative agent. Acute bronchitis can be defined as acute bacterial or viral infection of the larger airways in healthy patients with no history of recurrent disease. It affects over 40 adults per 1000 each year and consists of transient inflammation of the major bronchi and trachea. Most often it is caused by viral infection and hence antibiotic therapy is not indicated in immunocompetent individuals. Viral bronchitis can sometimes be treated using antiviral medications depending on the virus causing the infection, and medications such as anti-inflammatory drugs and expectorants can help mitigate the symptoms. Treatment of acute bronchitis with antibiotics is common but controversial as their use has only moderate benefit weighted against potential side effects (nausea and vomiting), increased resistance, and cost of treatment in a self-limiting condition. Beta2 agonists are sometimes used to relieve the cough associated with acute bronchitis. In a recent systematic review it was found there was no evidence to support their use.

"M. pneumoniae" infections can be differentiated from other types of pneumonia by the relatively slow progression of symptoms. A positive blood test for cold-hemagglutinins in 50–70% of patients after 10 days of infection (cold-hemagglutinin-test should be used with caution or not at all, since 50% of the tests are false-positive), lack of bacteria in a Gram-stained sputum sample, and a lack of growth on blood agar.

PCR has also been used.

Pleural empyema is a collection of pus in the pleural cavity caused by microorganisms, usually bacteria. Often it happens in the context of a pneumonia, injury, or chest surgery. It is one of various kinds of pleural effusion. There are three stages: exudative, when there is an increase in pleural fluid with or without the presence of pus; fibrinopurulent, when fibrous septa form localized pus pockets; and the final organizing stage, when there is scarring of the pleura membranes with possible inability of the lung to expand. Simple pleural effusions occur in up to 40% of bacterial pneumonias. They are usually small and resolve with appropriate antibiotic therapy. If however an empyema develops additional intervention is required.

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.