Pneumonia is inflammation of the lungs with consolidation or interstitial lung infiltrates, most often categorized according to the causative organism. Typical symptoms might include fever, cough, dyspnea, and chest pain. Because each specific type of pneumonia may result from a different etiology and pathogenic mechanism, each subtype also has its characteristic risk factors, signs, and symptoms.
Community-acquired pneumonia (CAP) is defined as pneumonia acquired outside hospital or healthcare facilities. Older patients in particular are often afebrile and may present with confusion and worsening of underlying diseases. The most common cause is Streptococcus pneumoniae (also known as pneumococcus), which is considered to be the prototype of typical bacterial pneumonia.  Mycoplasma pneumoniae is also a major cause and is considered to be the prototype of atypical bacterial pneumonia.
Hospital-acquired pneumonia (HAP) is an acute lower respiratory tract infection that is by definition acquired after at least 48 hours of admission to hospital and is not incubating at the time of admission. The spectrum of HAP is now distinct from ventilator-associated pneumonia (VAP), which is defined as pneumonia occurring more than 48 hours after endotracheal intubation. Healthcare-associated pneumonia (HCAP) is no longer considered a clinical entity in the most recent guidelines for HAP and VAP by the Infectious Diseases Society and the American Thoracic Society.  Guidelines also recommend that each hospital generate antibiograms to guide choice of antibiotics.
Viruses can cause pneumonia and present as atypical pneumonia, and are also a rare cause of hospital-acquired pneumonia in immunocompetent adults. Viral pneumonia is more common in children than in adults. Common community-acquired viral pathogens include influenza virus, respiratory syncytial virus (RSV), and parainfluenza virus. Influenza virus can cause severe pneumonia and can predispose to superinfection with Staphylococcus aureus leading to high mortality in young adults. Pneumonia due to cytomegalovirus (CMV) or varicella zoster virus (VZV) can be seen in immunocompromised hosts including those with HIV and is related to reactivation of prior infection. Less common causes of viral pneumonia include hantavirus, avian influenza, and SARS.
Severe acute respiratory syndrome (SARS) is a viral pneumonia caused by the SARS coronavirus (SARS-CoV).  Risk factors include history of recent travel, close contact with infected individuals, and laboratory work with SARS-CoV. Patients initially develop influenza-like prodromal symptoms. Cough (initially dry), dyspnea, and diarrhea may be present in the first week but are more commonly reported in the second week of illness. In severe cases, patients develop rapidly progressing respiratory distress and oxygen desaturation, with about 20% requiring intensive care.     Transmission occurs mainly during the second week of illness.
Middle East respiratory syndrome (MERS) is an acute viral respiratory tract infection caused by the novel betacoronavirus Middle East respiratory syndrome coronavirus (MERS-CoV). MERS should be considered when a severe respiratory illness occurs in the 2 weeks following residence in or travel to the Middle East or areas of outbreak, and/or close contact with infected individuals. The majority of cases are the result of human-to-human transmission with peaks of confirmed cases occurring during nosocomial outbreaks. The clinical spectrum of infection varies from no symptoms or mild respiratory symptoms to severe, rapidly progressive pneumonia, acute respiratory distress syndrome, septic shock, or multi-organ failure resulting in death. Treatment is mainly supportive.
Atypical bacterial pneumonia is caused by atypical organisms that are not detectable on Gram stain and cannot be cultured using standard methods. The most common organisms are Mycoplasma pneumoniae , Chlamydophila pneumoniae ( Chlamydia pneumoniae ), and Legionella pneumophila .  Atypical bacterial pneumonia is usually characterized by a symptom complex that includes headache, low-grade fever, cough, and malaise. Constitutional symptoms often predominate over respiratory findings. Although in most cases presentation can be in the milder spectrum of community-acquired pneumonia, some cases, especially if caused by L pneumophila , may present as severe pneumonia, necessitating intensive care admission.
Mycoplasma pneumoniae causes community-acquired pneumonia and upper respiratory illness. Occurs mainly in children and young adults, and is often seen in close community settings (e.g., boarding schools, army bases, and universities).
Chlamydia pneumoniae is a frequent respiratory pathogen in humans that occurs worldwide and in all age groups. It is a major cause of community-acquired pneumonia. Pneumonia due to C pneumoniae cannot be differentiated clinically from other atypical pneumonia-causing organisms, especially Mycoplasma pneumoniae . 
Legionella pneumonia, known as Legionnaires' disease, occurs when the bacteria are inhaled (or rarely aspirated) into the lungs. Presentation includes respiratory symptoms such as cough (may not be productive) and SOB, fever, chills, and chest pain. Other symptoms include headache, nausea, vomiting, abdominal pain, or diarrhea.
Pneumocystis pneumonia (PCP) is an infection of the lung caused by the fungal organism Pneumocystis jirovecii (formerly known as Pneumocystis carinii ). Typically, it causes clinical disease in severely immunocompromised patients, such as HIV-positive patients with CD4 cell counts <200 cells/microlitre, bone marrow transplant patients, solid-organ transplant patients, or patients on chronic immunosuppressive therapy.
Coccidioidomycosis is a fungal infection caused by the endemic fungus Coccidioides species and acquired through inhalation of airborne arthrospores within the endemic areas of the southwest US, northern Mexico, and limited areas of Central and South America. Coccidioidomycosis may be asymptomatic or can cause acute and chronic pulmonary syndromes, and, rarely, extrapulmonary infection.
Invasive aspergillosis is caused by filamentous fungi of the Aspergillus species, which are found ubiquitously in soil. Inhalation of the aerosolized conidia (spores) causes the infection. Mostly affects immunocompromised patients; it is rare in immunocompetent hosts. Clinical findings are nonspecific and include fever, cough, and pleuritic pain. High index of suspicion is required for early diagnosis. Lungs, sinuses, brain, and skin are sites of involvement. Aspergilloma forms in preformed lung cavities. It is usually asymptomatic.
Aspiration is the inhalation of foreign material into the airways beyond the vocal cords.  It can be categorized as aspiration pneumonitis or aspiration pneumonia. Aspiration pneumonitis is chemical injury after aspiration of gastric contents.
Aspiration pneumonia results from inhalation of oropharyngeal contents into the lower airways, leading to lung injury and resultant bacterial infection. Key diagnostic factors include cough and dyspnea. Strong risk factors include altered mental status (e.g., reduced level of consciousness may lead to inadequate cough reflex and impaired glottal closure), swallowing dysfunction (e.g., in stroke patients), upper GI disease, intubation or tracheostomy tube, feeding tube, older age, and recumbent position.  
Neonatal pneumonia may result from aspiration, often of meconium. 
Bronchiolitis obliterans organising pneumonia (BOOP) is defined as organized polypoid granulation tissue in the distal airways extending into the alveolar ducts and alveoli.    The typical presentation of idiopathic BOOP is an individual with a flu-like illness consisting of a mild fever, arthralgia, fatigue, and a mild cough lasting 1 to 3 months. Shortness of breath develops later as the BOOP occupies increasing numbers of alveoli. Bilateral end-inspiratory crackles are heard. Other presentations depend on the type. BOOP may occur after infectious pneumonias. Organisms include viral agents, bacterial agents, atypical organisms, and parasites. 
Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis, is the result of non-IgE mediated immunologic inflammation. HP is caused by repeated inhalation of nonhuman protein, which can be of natural plant or animal origin or can be the result of a chemical conjugated to a human airway protein, such as albumin. The inflammation of HP manifests itself in the alveoli and distal bronchioles. The clinical manifestations of HP depend on the concentration and frequency of exposure. The clinical syndromes (acute, subacute, and chronic HP) present differently.
Persistent pulmonary infiltrate results when a substance denser than air (e.g., pus, oedema, blood, surfactant, protein, or cells) lingers within the lung parenchyma. Nonresolving and slowly resolving pneumonias are the most common broad categories of persistent pulmonary infiltrate. Persistence is attributed to defects in host immune defense mechanisms, presence of unusual or resistant organisms, or diseases that mimic pneumonia.
Dyspnea, also known as shortness of breath or breathlessness, is a subjective sensation of breathing discomfort. The etiology is broad, ranging from mild, self-limiting processes to life-threatening conditions. Diseases of the cardiovascular, pulmonary, and neuromuscular systems are the most common etiologies. Dyspnea is a key diagnostic factor of pneumonia.
Cough is the most common presenting symptom in primary practice.  Postinfectious cough is the most common etiology of subacute cough (cough persisting for 3 to 8 weeks).  Once cough duration has exceeded 8 weeks, a systematic approach to elucidating cause and best treatment is needed. Common etiologies of chronic cough (cough persisting for >8 weeks) in nonsmoking adults with a normal CXR who do not take ACE inhibitors include upper airway cough syndrome, asthma, gastroesophageal reflux disease, and nonasthmatic eosinophilic bronchitis.    Patients with chronic cough (usually productive of sputum), a history of fever, malaise, and chest pain, and exam findings of dullness to percussion, decreased breath sounds, and presence of rales, should be further tested for pneumonia.
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This overview has been compiled using the information in existing sub-topics.
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