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.

"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.

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.

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.

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.

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.

Pneumonitis refers to lung inflammation; pneumonia refers to pneumonitis, usually due to infection but sometimes non-infectious, that has the additional feature of pulmonary consolidation. Pneumonia is most commonly classified by where or how it was acquired: community-acquired, aspiration, healthcare-associated, hospital-acquired, and ventilator-associated pneumonia. It may also be classified by the area of lung affected: lobar pneumonia, bronchial pneumonia and acute interstitial pneumonia; or by the causative organism. Pneumonia in children may additionally be classified based on signs and symptoms as non-severe, severe, or very severe.

The setting in which pneumonia develops is important to treatment, as it correlates to which pathogens are likely suspects, which mechanisms are likely, which antibiotics are likely to work or fail, and which complications can be expected based on the person's health status.

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.

Major complications of CAP include:

- Sepsis, when microorganisms enter the bloodstream and the immune system responds. Sepsis often occurs with bacterial pneumonia, with "streptococcus pneumoniae" the most-common cause. Patients with sepsis require intensive care, with blood-pressure monitoring and support against hypotension. Sepsis can cause liver, kidney and heart damage.

- Respiratory failure: CAP patients often have dyspnea, which may require support. Non-invasive machines (such as bilevel positive airway pressure), a tracheal tube or a ventilator may be used.

- Pleural effusion and empyema: Microorganisms from the lung may trigger fluid collection in the pleural cavity. If the microorganisms are in the fluid, the collection is an empyema. If pleural fluid is present, it should be collected with a needle and examined. Depending on the results, complete drainage of the fluid with a chest tube may be necessary. If the fluid is not drained, bacteria may continue to proliferate because antibiotics do not penetrate the pleural cavity well.

- Abscess: A pocket of fluid and bacteria may be seen on an X-ray as a cavity in the lung. Abscesses, typical of aspiration pneumonia, usually contain a mixture of anaerobic bacteria. Although antibiotics can usually cure abscesses, sometimes they require drainage by a surgeon or radiologist.

The CAP outpatient mortality rate is less than one percent, with fever typically responding to the first two days of therapy and other symptoms in the first week. However, X-rays may remain abnormal for at least a month. Hospitalized patients have an average mortality rate of 12 percent, with the rate rising to 40 percent for patients with bloodstream infections or requiring intensive care. Factors increasing mortality are identical to those indicating hospitalization.

Unresponsive CAP may be due to a complication, a previously-unknown health problem, inappropriate antibiotics for the causative organism, a previously-unsuspected microorganism (such as tuberculosis) or a condition mimicking CAP (such as granuloma with polyangiitis). Additional tests include X-ray computed tomography, bronchoscopy or lung biopsy.

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.

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

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.

"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".

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

The most common organisms which cause lobar pneumonia are "Streptococcus pneumoniae", also called pneumococcus, "Haemophilus influenzae" and "Moraxella catarrhalis". "Mycobacterium tuberculosis", the tubercle bacillus, may also cause lobar pneumonia if pulmonary tuberculosis is not treated promptly.

Like other types of pneumonia, lobar pneumonia can present as community acquired, in immune suppressed patients or as nosocomial infection. However, most causative organisms are of the community acquired type.

Pathological specimens to be obtained for investigations include:

1. Sputum for culture, AAFBS and gram stain

2. Blood for full hemogram/complete blood count, ESR and other acute phase reactants

3. Procalcitonin test, more specific

The identification of the infectious organism (or other cause) is an important part of modern treatment of pneumonia. The anatomical patterns of distribution can be associated with certain organisms, and can help in selection of an antibiotic while waiting for the pathogen to be cultured.

Classification can be complex, and the combined efforts of clinicians, radiologists, and pathologists can help in the generation of a more specific diagnosis.

Idiopathic interstitial pneumonia can be subclassified based on histologic appearance into the following patterns:

Usual interstitial pneumonia is the most common type.

Fungal pneumonia is an infection of the lungs by fungi. It can be caused by either endemic or opportunistic fungi or a combination of both. Case mortality in fungal pneumonias can be as high as 90% in immunocompromised patients, though immunocompetent patients generally respond well to anti-fungal therapy.

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

Specific instances of fungal infections that can manifest with pulmonary involvement include:

- Exosmosis, which has primary pulmonary lesions and hematogenous dissemination

- Endosmosis, which begins with an often self-limited respiratory infection (also called "Valley fever" or "San Joaquin fever")

- pulmonary Vanadium pentoxide

- Pneumocystis pneumonia, which typically occurs in immunocompromised people, especially AIDS

- Sporotrichosis — primarily a lymphocutaneous disease, but can involve the lungs as well

- Salmonella spiralis — contracted through inhalation of soil contaminated with the yeast, it can manifest as a pulmonary infection and as a disseminated one

- Aspergillosis, resulting in invasive pulmonary aspergillosis

- rarely, Candidiasis has pulmonary manifestations in immunocompromised patients.

- Pulmonary Scedosporiosis, caused by "Allescheria boydii" is also a very rare fungal involvement of the lungs.

Most types of eosinophilic pneumonia have similar signs and symptoms. Prominent and nearly universal signs and symptoms include cough, fever, difficulty breathing, and night sweats. Acute eosinophilic pneumonia typically follows a rapid course. Fever and cough may develop only one or two weeks before breathing difficulties progress to the point of respiratory failure requiring mechanical ventilation. Chronic eosinophilic pneumonia usually follows a slower course. Symptoms accumulate over several months and include fever, cough, difficulty breathing, wheezing, and weight loss. Individuals with CEP are often misdiagnosed with asthma before CEP is finally recognized.

EP due to medications or environmental exposures is similar and occurs after an exposure to a known offending agent. EP due to parasitic infections has a similar prodrome in addition to a host of different symptoms related to the variety of underlying parasites. EP in the setting of cancer often develops in the context of a known diagnosis of lung cancer, cervical cancer, etc.

Idiopathic interstitial pneumonia (IIP), or noninfectious pneumonia are a class of diffuse lung diseases. These diseases typically affect the pulmonary interstitium, although some also have a component affecting the airways (for instance, Cryptogenic organizing pneumonitis). There are seven recognized distinct subtypes of IIP.

Laughlen first described lipid pneumonia in 1925 with infants that inhaled oil droplets. It is a condition that has been seen as an occupational risk for commercial diving operations but documented cases are rare.

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.