Classification of pneumonia

Community-acquired pneumonia (CAP) is infectious pneumonia in a person who has not recently been hospitalized. CAP is the most common type of pneumonia. The most common causes of CAP vary depending on a person's age, but they include Streptococcus pneumoniae, viruses, the atypical bacteria, and Haemophilus influenzae. Overall, Streptococcus pneumoniae is the most common cause of community-acquired pneumonia worldwide. Gram-negative bacteria cause CAP in certain at-risk populations. CAP is the fourth most common cause of death in the United Kingdom and the sixth in the United States. The term "walking pneumonia" has been used to describe a type of community-acquired pneumonia of less severity (because the sufferer can continue to "walk" rather than requiring hospitalization).[2] Walking pneumonia is usually caused by the atypical bacterium, Mycoplasma pneumoniae.[3]

Hospital-acquired pneumonia, also called nosocomial pneumonia, is pneumonia acquired during or after hospitalization for another illness or procedure with onset at least 72 hrs after admission. The causes, microbiology, treatment and prognosis are different from those of community-acquired pneumonia. Up to 5% of patients admitted to a hospital for other causes subsequently develop pneumonia. Hospitalized patients may have many risk factors for pneumonia, including mechanical ventilation, prolonged malnutrition, underlying heart and lung diseases, decreased amounts of stomach acid, and immune disturbances. Additionally, the microorganisms a person is exposed to in a hospital are often different from those at home. Hospital-acquired microorganisms may include resistant bacteria such as MRSA, Pseudomonas, Enterobacter, and Serratia. Because individuals with hospital-acquired pneumonia usually have underlying illnesses and are exposed to more dangerous bacteria, it tends to be more deadly than community-acquired pneumonia. Ventilator-associated pneumonia (VAP) is a subset of hospital-acquired pneumonia. VAP is pneumonia which occurs after at least 48 hours of intubation and mechanical ventilation.

Bronchiolitis obliterans organizing pneumonia (BOOP) is caused by inflammation of the small airways of the lungs. It is also known as cryptogenic organizing pneumonitis (COP).

Eosinophilic pneumonia

Eosinophilic pneumonia is invasion of the lung by eosinophils, a particular kind of white blood cell. Eosinophilic pneumonia often occurs in response to infection with a parasite or after exposure to certain types of environmental factors.

Chemical pneumonia (usually called chemical pneumonitis) is caused by chemical toxicants such as pesticides, which may enter the body by inhalation or by skin contact. When the toxic substance is an oil, the pneumonia may be called lipoid pneumonia.

Aspiration pneumonia (or aspiration pneumonitis) is caused by aspirating foreign objects which are usually oral or gastric contents, either while eating, or after reflux or vomiting which results in bronchopneumonia. The resulting lung inflammation is not an infection but can contribute to one, since the material aspirated may contain anaerobic bacteria or other unusual causes of pneumonia. Aspiration is a leading cause of death among hospital and nursing home patients, since they often cannot adequately protect their airways and may have otherwise impaired defenses.

Dust pneumonia describes disorders caused by excessive exposure to dust storms, particularly during the Dust Bowl in the United States. With dust pneumonia, dust settles all the way into the alveoli of the lungs, stopping the cilia from moving and preventing the lungs from ever clearing themselves.

Necrotizing pneumonia (NP), also known as cavitary pneumonia or cavitatory necrosis, is a rare but severe complication of lung parenchymal infection.[5][6][7] In necrotizing pneumonia, there is a substantial liquefaction following death of the lung tissue, which may lead to gangrene formation in the lung.[8][9] In most cases patients with NP have fever, cough and bad breath, and those with more indolent infections have weight loss.[10] Often patients clinically present with acute respiratory failure.[10] The most common pathogens responsible for NP are Streptococcus pneumonia, Staphylococcus aureus, Klebsiella pneumoniae.[11]

People with weakened immune defense, such as HIV/AIDS patients, are highly susceptible to opportunistic infections affecting the lungs.[12] Most common pathogens are Pneumocystis jiroveci, Mycobacterium avium-intracellulare complex, Streptococcus pneumoniae, Haemophilus species. Less frequent pathogens are Cryptococcus neoformans, Histoplasma capsulatum, Coccidioides immitis, cytomegalovirus (CMV), and Toxoplasma gondii.[13]

Chemotherapy-induced immunodeficiency may lead to severe lung infections.[14] Pathogens commonly associated with lung infectioins are bacteria (like Pseudomonas aeruginosa, Stenotrophomonas maltophilia, and Nocardia species), viruses (eg, respiratory syncytial virus, parainfluenza virus, influenza virus A and influenza B, and cytomegalovirus), and fungi (eg, Aspergillus, Fusarium, and Mucorales species, and Pneumocystis jirovecii).[14]

This is a historical term for acute lung injury (ALI) or acute respiratory distress syndrome (ARDS).[15] However, the term was and, especially by lay people, still is used to denote pneumonia affecting both lungs. Accordingly, the term 'double pneumonia' is more likely to be used to describe bilateral pneumonia than it is ALI or ARDS.

Severe acute respiratory syndrome (SARS) is a highly contagious and deadly type of pneumonia which first occurred in Nov-2002 after initial outbreaks in China caused by SARS-CoV/SARS-CoV-1,which almost disappeared by the month of May-2004. The second outbreak of SARS-CoV-2 started in December 2019 from Wuhan, China and was declared pandemic by WHO on 11 March 2020. SARS is caused by the SARS coronavirus, a previously unknown pathogen.