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.
Pneumonia in a patient who has not been hospitalized and has not been resident in a long-term care facility (such as a nursing home) within the past 14 days. 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) and ventilator-associated pneumonia (VAP) have been defined as 2 separate groups.  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 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) 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. Clinical presentation ranges from asymptomatic to severe, rapidly progressive, potentially fatal pneumonia. Treatment is mainly supportive.
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 .  Typically, is characterized by a symptom complex that includes headache, low-grade fever, cough, and malaise. Constitutional symptoms often predominate over respiratory findings.
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).
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 . 
Acquired by inhalation of aerosolized bacteria or, rarely, microaspiration of contaminated drinking water. 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.
Caused by the fungal organism Pneumocystis jirovecii (formerly P carinii ). Key diagnostic factors include HIV or HIV risk factors and low CD4 cell count. It is the most common AIDS-defining illness. Strong risk factors include CD4 count <200 cells/microliter (e.g., HIV-positive patients), immunosuppressed state (e.g., organ transplant, hematologic malignancies), chronic corticosteroid therapy, and history of prior pneumocystis pneumonia.
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.
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.  
Defined as organized polypoid granulation tissue in the distal airways extending into the alveolar ducts and alveoli.    The typical presentation of idiopathic bronchiolitis obliterans organizing pneumonia (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. 
Also known as extrinsic allergic alveolitis, it is the result of non-IgE mediated immunologic inflammation. Hypersensitivity pneumonitis (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.
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 is indicative of respiratory distress and is a key diagnostic factor of pneumonia. 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.
Cough is the most common presenting symptom in primary practice.  Postinfectious cough is the most common etiology of subacute cough.  Other common etiologies 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 suspected and further tested for pneumonia.
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This overview has been compiled using the information in existing sub-topics.
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