Pneumonia causes

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: , Hamid Qazi, MD, BSc [2], Priyamvada Singh, M.D. [3]; Alejandro Lemor, M.D. [4]; Ogheneochuko Ajari, MB.BS, MS [5]

Overview

Pneumonia can result from a variety of causes, including infection with bacteria, viruses, fungi, parasites, and chemical or physical injury to the lungs. The etiology will depend upon various factors such as age, immune status, geographical area, and comorbidities.

Causes

Life Threatening Causes

Life-threatening causes include conditions which may result in death or permanent disability within 24 hours if left untreated.

• Anthrax

Common Causes

• Cryptococcus neoformans
• Flu
• Histoplasmosis
• Hospital-acquired pneumonia
• Mycoplasma pneumonia
• Pneumocystis jiroveci
• Respiratory syncytial virus
• Rhinovirus
• Streptococcus pneumoniae

Causes by Organ System

Cardiovascular	Austrian triad, Dressler syndrome
Chemical/Poisoning	Ammonia inhalation, chlorine gas, chloroacetophenone, CS gas, formalin vapors inhalation, gold salts, gold, hydrocarbon poisoning, hydrogen sulfide, nitrogen dioxide, ozone inhalation, phosgene inhalation, sulfur dioxide inhalation
Dental	No underlying causes
Dermatologic	Chediak-Higashi syndrome, dermatomyositis, Lenz-Majewski hyperostotic dwarfism syndrome, systemic lupus erythematosus , zygomycosis
Drug Side Effect	Acebutolol, amiodarone, belimumab, bleomycin, blinatumomab, boceprevir, bucillamine, captopril, carbamazepine, ceritinib, crizotinib, cyclophosphamide, dihydroergocryptine, dihydroergotamine, docetaxel, dornase alfa, dronedarone, enfuvirtide,ethanolamine oleate, felbamate, gemcitabine, hexamethonium, idelalisib, iloperidone, interferon alfacon-1, mefloquine, mesalazine, methotrexate, minocycline, nilutamide, nitrofurantoin, nivolumab, olaparib, paclitaxel, pegylated interferon alfa-2b, pembrolizumab, penicillamine, phenytoin, rituximab, sulfasalazine, ticlopidine, trametinib
Ear Nose Throat	Epiglottitis, laryngeal cleft
Endocrine	No underlying causes
Environmental	Asbestosis, brucella abortus, byssinosis, extrinsic allergic alveolitis, occupational lung disease, pneumoconiosis, talc pneumoconiosis
Gastroenterologic	Achalasia, cystic fibrosis, esophageal atresia, gastroesophageal reflux, infantile dysphagia, inflammatory bowel disease, zygomycosis
Genetic	Aicardi syndrome, ataxia telangiectasia, Becker's muscular dystrophy, Chediak-Higashi syndrome, chronic granulomatous disease, congenital pulmonary anomalies, Cornelia de Lange syndrome, cystic fibrosis, Down syndrome, Duchenne muscular dystrophy, Jeune's syndrome, Kartagener syndrome, Machado-Joseph disease, Marie type ataxia, Nezelof syndrome, osteogenesis imperfecta, pulmonary surfactant-associated protein C, SCID, Simpson-Golabi-Behmel syndrome, WHIM syndrome, x-linked agammaglobulinemia
Hematologic	Agranulocytosis, atransferrinemia, infantile hypophosphatasia, multiple myeloma, neutropenia, pseudophosphatasia, sickle cell anemia, x-linked agammaglobulinemia
Iatrogenic	Anesthesia, general anaesthesia, radiotherapy
Infectious Disease	Achromobacter xylosoxidans, acinetobacter baumannii, actinomycosis, acute bronchitis, acute upper respiratory infection, acute viral nasopharyngitis (common cold), adenovirus, adiaspiromycosis, allergic bronchopulmonary aspergillosis, anaerobes, anthrax, aspergillosis, aspergillus, aspiration pneumonia, aspiration, atypical pneumonia, aureobasidium pullulans exposure, Austrian triad, avian influenza, bacillus anthracis, blastomyces dermatitidis, blastomycosis, bocavirus, bordetella pertussis, brucella abortus, burkholderia cepacia, burkholderia pseudomallei, candida, chemical pneumonia, chicken pox, chlamydia trachomatis, chlamydophila pneumoniae, chlamydophila psittaci, coccidioides immitis, coccidioidomycosis, common cold, community-acquired pneumonia, coronavirus, coxiella burnetii, cryptococcus neoformans, cytomegalovirus, encephalitozoon cuniculi infection, enterobacter, enterovirus, eosinophilic pneumonia, epiglottitis, escherichia coli, flu, francisella tularensis, gram-negative bacilli, Group A streptococcal infection, Group B streptococcal infection, haemophilus influenzae, hantavirus, herpes simplex virus, histoplasma capsulatum, histoplasmosis, HIV, human T-lymphotropic virus, hypersensitivity pneumonitis, influenza, Kartagener syndrome, klebsiella pneumoniae, klebsiella, Kyasanur-Forrest disease, legionella pneumophila, Legionnaires' disease, leptospira, listeria monocytogenes, listeriosis, lung abscess, malignant buotonneuse fever, measles, melioidosis, metapneumovirus, methicillin-resistant staphylococcus aureus, Middle East respiratory syndrome coronavirus infection, moraxella catarrhalis, MSSA, mumps, mycobacterium avium-intracellulare, mycobacterium haemophilum, mycobacterium kansasii, mycobacterium tuberculosis, mycoplasma hominis, mycoplasma pneumoniae, neisseria meningitidis, nocardia, nocardiosis, paracoccidioides, paragonimiasis, parainfluenza, parechovirus, pertussis, plague, pleurisy, pneumococcal pneumonia, pneumococcus, pneumocystis jiroveci, pneumocystis jirovecii pneumonia, pneumonic plague, proteus, pseudomonas aeruginosa, pseudomonas pseudomallei, psittacosis, Q fever, rat-bite fever, respiratory syncytial virus, rheumatic fever, rhinovirus, rhodococcus equi, rickettsia rickettsii, serratia, severe acute respiratory syndrome, sporotrichosis, staphylococcus aureus, stenotrophomonas maltophilia, sterile pneumonitis, streptococcus agalactiae, streptococcus group A, streptococcus pneumoniae, swine flu, torulopsis, toxocariasis, toxoplasma gondii, treponema pallidum, trichosporon, trypanosomiasis, tularemia, typhoid fever, typhus, ureaplasma urealyticum, varicella, varicella-zoster virus, whooping cough, yersinia enterocolitica, yersinia pestis, zygomycosis
Musculoskeletal/Orthopedic	Becker's muscular dystrophy, Duchenne muscular dystrophy, idiopathic myopathy, Jeune's syndrome, Lenz-Majewski hyperostotic dwarfism syndrome, osteogenesis imperfecta, polymyositis, skeletal dysplasia
Neurologic	Aicardi syndrome, Alzheimer's disease, Arnold-Chiari malformation, ataxia telangiectasia, cerebrovascular accident, corticobasal degeneration, idiopathic myopathy, idiopathic Parkinson's disease, Machado-Joseph disease, Marie type ataxia, Mobius syndrome, neisseria meningitidis, Parkinson's disease, persistent vegetative state, precocious myoclonic encephalopathy, progressive supranuclear palsy
Nutritional/Metabolic	Pulmonary surfactant-associated protein C
Obstetric/Gynecologic	Gastric content aspiration
Oncologic	Bronchogenic carcinoma, lung cancer, malignancy, multiple myeloma
Ophthalmologic	Aicardi syndrome, multiple pterygium syndrome
Overdose/Toxicity	Chemical pneumonia, cocaine abuse
Psychiatric	Alzheimer's disease, cocaine abuse, corticobasal degeneration
Pulmonary	Acute bronchitis, acute interstitial pneumonia, acute respiratory distress syndrome, acute upper respiratory infection, acute viral nasopharyngitis (common cold), allergic bronchopulmonary aspergillosis, asbestosis, aspiration pneumonia, aspiration, atypical pneumonia, Austrian triad, bronchiectasis, bronchogenic carcinoma, byssinosis, chemical pneumonia, cholesterol pneumonia, common cold, community-acquired pneumonia, congenital pulmonary anomalies, cryptogenic organizing pneumonia, eosinophilic pneumonia, extrinsic allergic alveolitis, flu, gastric content aspiration, Hamman-Rich syndrome,hospital-acquired pneumonia hypersensitivity pneumonitis, interstitial lung disease, Kartagener syndrome, Legionnaires' disease, lipoid pneumonia, Löffler's syndrome, lung abscess, lung cancer, lymphocytic interstitial pneumonia, mycobacterium tuberculosis, neonatal pneumonia, Pittsburgh pneumonia, pleurisy, pneumococcal pneumonia, pneumoconiosis, severe acute respiratory syndrome, sterile pneumonitis, usual interstitial pneumonia, whooping cough, Williams-Campbell syndrome
Renal/Electrolyte	Chronic renal failure, systemic lupus erythematosus
Rheumatology/Immunology/Allergy	Allergic bronchopulmonary aspergillosis, byssinosis, chronic granulomatous disease, common variable immunodeficiency, dermatomyositis, extrinsic allergic alveolitis, hyper-IgE syndrome, hyper-IgM syndrome, hypersensitivity pneumonitis, IgG deficiency, immunoglobulin G subclass deficiency, Nezelof syndrome, polymyositis, rheumatic fever, rheumatoid arthritis, SCID, Sjogren's syndrome, Still's disease, systemic lupus erythematosus, systemic sclerosis, WHIM syndrome, x-linked agammaglobulinemia
Sexual	No underlying causes
Trauma	No underlying causes
Urologic	No underlying causes
Miscellaneous	No underlying causes

Causes

Most Common Etiologies for Community-Acquired Pneumonia

Outpatient	Inpatient (non-ICU)	Inpatient (ICU)
Streptococcus pneumoniae Mycoplasma pneumoniae Haemophilus influenzae Chlamydophila pneumoniae Influenza A and B, adenovirus, respiratory syncytial virus, parainfluenza	Streptococcus pneumoniae Mycoplasma pneumoniae Haemophilus influenzae Legionella Aspiration Influenza A and B, adenovirus, respiratory syncytial virus, parainfluenza Yersinia enterocolitica	Streptococcus pneumoniae Staphylococcus aureus Legionella Gram-negative bacilli Haemophilus influenzae Acinetobacter baumannii

Common Causes by Age Group

Age Group	Neonates	Children	Adults
Bacteria	Listeria monocytogenes Mycobacterium tuberculosis	Mycoplasma pneumoniae Chlamydophila pneumoniae Legionella pneumophila Chlamydia trachomatis	Typical Bacteria Streptococcus pneumoniae Haemophilus influenzae Escherichia coli Klebsiella pneumoniae Pseudomonas aeruginosa Atypical Bacteria Mycoplasma pneumoniae Chlamydophila pneumoniae Legionella pneumophila
Virus	Herpes simplex virus Adenovirus Mumps Enterovirus	Respiratory syncytial virus (RSV) Metapneumovirus Adenovirus Parainfluenza Influenza Rhinovirus	Influenza Parainfluenza Respiratory syncytial virus (RSV) Metapneumovirus Adenovirus

• Newborn infants, children, and adults are at risk for different spectrums of disease causing microorganisms.

• In addition, adults with chronic illnesses, who live in certain parts of the world, who reside in nursing homes, who have recently been treated with antibiotics, or who are alcoholics are at risk for unique infections.

Infants and Newborns

• GBS causes at least 50% of cases of CAP in the first week of life.
• Other bacterial causes in the newborn period include Listeria monocytogenes and Mycobacterium tuberculosis.
• Viral causes like herpes simplex virus (most common), adenovirus, mumps, and enterovirus.

Children

• For the most part, children older than one month are at risk for the same microorganisms as adults.
• Children less than five years are much less likely to have pneumonia caused by Mycoplasma pneumoniae, Chlamydophila pneumoniae, or Legionella pneumophila.
• In contrast, older children and teenagers are more likely to acquire Mycoplasma pneumoniae and Chlamydophila pneumoniae than adults.
• A unique cause of CAP in this group is Chlamydia trachomatis, which is acquired during birth but does not cause pneumonia until 2-4 weeks later.
• Common viruses include respiratory syncytial virus (RSV), metapneumovirus, adenovirus, parainfluenza, influenza, and rhinovirus.
• RSV in particular is a common source of illness and hospitalization.
• Fungi and parasites are not typically encountered in otherwise healthy infants, though maternally-derived syphilis can be a cause of CAP in this age group.

Adults

The causes of CAP in adults are outlined in the following categories.

Viruses

• Viruses account for about 20% cases of CAP.
• Common viruses are influenza, parainfluenza, respiratory syncytial virus, metapneumovirus, and adenovirus.
• Less common viruses include chicken pox, SARS, avian flu, and hantavirus.

Streptococcus pneumoniae

• Streptococcus pneumoniae is the most common cause of community-acquired pneumonia.
• Aspiration pneumonia is most commonly caused by anaerobic organisms.
• Prior to the development of antibiotics and vaccination, it was a leading cause of death.
• Traditionally, it was highly sensitive to penicillin, but during the 1970s resistance to multiple antibiotics began to develop.
• Current strains of drug resistant Streptococcus pneumoniae (DRSP) are common, accounting for twenty percent of all streptococcal infections.
• risk factors for DRSP in adults include: being older than 65, having exposure to children in day care, alcoholism, other severe underlying disease, or recent treatment with antibiotics; individuals exposed to these risk factors should initially be treated with antibiotics effective against DRSP.

Atypical Organisms

• Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila are often grouped as atypical pneumonia. Community-acquired pneumonia caused by these agents present insidiously, with a non-productive cough and prominent extra-pulmonary complaints, such as myalgias and diarrhea (lack the typical pneumonia symptoms of fever, cough, and sputum).

• Mycoplasma pneumoniae is often referred to as "walking pneumonia." It is transmitted via respiratory droplets and is common among healthy individuals in close contact with one another in settings such as dormitories or military barracks.

• Atypical organisms are more difficult to grow and respond to different antibiotics; they were discovered more recently than the typical bacteria discovered in the early twentieth century.

Haemophilus influenzae

• Haemophilus influenzae used to be a common bacterial cause of CAP.
• First discovered in 1892, it was initially believed to be the cause of influenza because it commonly causes CAP in people who have suffered recent lung damage from viral pneumonia.

Enteric Gram-Negative Bacteria

• Enteric bacteria such as Escherichia coli and Klebsiella pneumoniae may cause commnity-acquired penumonia.
• risk factors in adults for infection include: living in a nursing home, serious heart and lung disease, and recent antibiotic use; these individuals should initially be treated with antibiotics effective against enteric Gram-negative bacteria.

Pseudomonas aeruginosa

• Pseudomonas aeruginosa is an uncommon cause of CAP, but it is a particularly difficult bacteria to treat.
• Individuals who are malnourished, have bronchiectasis, are on corticosteroids, or have recently had strong antibiotics for a week or more, should initially be treated with antibiotics effective against Pseudomonas aeruginosa.

Special Situations

• Coccidioides spp. are common in southwestern area of the United States.
• Anaerobic infection is common in alcoholics. Pneumococcal pneumonia remains the most common cause of CAP in alcoholics.
• Psittacosis (caused by Chlamydophila psittaci) should be considered in the patient with exposure to birds or bird droppings.
• Anaerobes are common in patients with poor dental hygiene and a suspected large volume of aspiration.
• Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Legionella species are the common causes of community acquired pneumonia in those with chronic obstructive pulmonary disorders and smokers.
• Streptococcus pneumoniae, Gram-negative bacilli, Haemophilus influenzae, Staphylococcus aureus, anaerobes, and Chlamydophila pneumoniae are more common in nursing home residents.
• Streptococcus pneumoniae, Haemophilus influenzae, and Mycobacterium tuberculosis are common pathogens in early stages of HIV, whereas, Pneumocystis jiroveci, Histoplasma, and Cryptococcus are commonly seen in late stages HIV.
• In patients with structural lung disease such as bronchiectasis and cystic fibrosis, Pseudomonas aeruginosa, Burkholderia cepacia, and Staphylococcus aureus are the common pathogens involved.

Aspiration Pneumonia

• Incompetent swallowing mechanism, as can be found in neurological disease (a common cause being strokes) or while a person is intoxicated.
• Iatrogenic causes such as general anaesthesia for an operation. Patients are therefore instructed to be nil per os (NPO) for at least four hours before surgery.
• Whether aspiration pneumonia represents a true bacterial infection or a chemical inflammatory process remains the subject of significant controversy.

Causes by Organ System

Cardiovascular	No underlying causes
Chemical/Poisoning	No underlying cause
Dental	No underlying causes
Dermatologic	No underlying causes
Drug Side Effect	No underlying cause
Ear Nose Throat	No underlying causes
Endocrine	No underlying causes
Environmental	No underlying cause
Gastroenterologic	No underlying cause
Genetic	No underlying cause
Hematologic	No underlying causes
Iatrogenic	No underlying cause
Infectious Disease	Acinetobacter baumannii, Actinomycosis, Adenovirus (serotype 1, 2, 3, 4, 5, 7, 14, 21, 35), AIDS-related opportunistic infections, Aspergillosis, Aspiration, Bacillus anthracis, Bird flu (avian influenza), Blastomycosis, Burkholderia cepacia, Burkholderia pseudomallei, Chlamydophila pneumoniae, Chickenpox, Coccidioides immitis, Coronavirus, Coxiella burnetii (Q fever), Cryptococcus neoformans, Cytomegalovirus, Escherichia coli, Enterovirus, Francisella tularensis, Gram-negative bacilli, Haemophilus influenzae, H1N1 flu, Hantavirus, Herpes simplex virus, Histoplasma capsulatum, Histoplasmosis, Human bocavirus, Human parechovirus types 1, 2, and 3, Influenza, Klebsiella pneumoniae, Legionella micdadei, Legionella pneumophila (Legionnaires' disease), Listeria monocytogenes, Mycoplasma pneumoniae, Measles, Melioidosis, Metapneumovirus, Middle East respiratory syndrome coronavirus, Moraxella catarrhalis, Mumps, Mycobacterium avium-intracellulare, Mycobacterium haemophilum, Mycobacterium kansasii, Neisseria meningitidis, New Haven coronavirus, Nocardia, Paragonimiasis, Parainfluenza, Pneumocystis jiroveci, Pneumonic plague, Proteus, Pseudomonas aeruginosa, Chlamydophila psittaci (psittacosis), Rat-bite fever, Respiratory syncytial virus (RSV), Rhinovirus, Rhodococcus equi, Streptococcus pneumoniae (pneumococcus), SARS, Serratia, Severe acute respiratory syndrome, Sporotrichosis , Staphylococcus aureus, Streptococcus agalactiae (group B streptococcus), Streptococcus pyogenes (group A streptococcus), Streptococcus pneumoniae, Swine flu, Toxocariasis , Toxoplasma gondii, Trichosporon , Trypanosomiasis, Typhus, Varicella-zoster virus, Yersinia enterocolitica, Yersinia pestis, Zygomycosis
Musculoskeletal/Orthopedic	No underlying causes
Neurologic	No underlying causes
Nutritional/Metabolic	No underlying cause
Obstetric/Gynecologic	No underlying causes
Oncologic	No underlying cause
Ophthalmologic	No underlying causes
Overdose/Toxicity	No underlying cause
Psychiatric	No underlying causes
Pulmonary	No underlying causes
Renal/Electrolyte	No underlying causes
Rheumatology/Immunology/Allergy	No underlying cause
Sexual	No underlying causes
Trauma	No underlying causes
Urologic	No underlying causes
Miscellaneous	No underlying cause

Causes in Alphabetical Order

• Acinetobacter baumannii

• Actinomycosis

• Adenovirus (serotype 1, 2, 3, 4, 5, 7, 14, 21, 35)

• AIDS-related opportunistic infections

• Aspergillosis

• Aspiration

• Bacillus anthracis

• Bird flu (avian influenza)

• Blastomycosis

• Burkholderia cepacia

• Burkholderia pseudomallei

• Chlamydophila pneumoniae

• Chickenpox

• Coccidioides immitis

• Coronavirus

• Coxiella burnetii (Q fever)

• Cryptococcus neoformans

• Cytomegalovirus

• Escherichia coli

• Enterovirus

• Francisella tularensis

• Gram-negative bacilli

• H1N1 flu

• Haemophilus influenzae

• Hantavirus

• Herpes simplex virus

• Histoplasma capsulatum

• Histoplasmosis

• Human bocavirus

• Human parechovirus types 1, 2, and 3

• Influenza

• Klebsiella pneumoniae

• Legionella micdadei

• Legionella pneumophila (Legionnaires' disease)

• Listeria monocytogenes

• Mycoplasma pneumoniae

• Measles

• Melioidosis

• Metapneumovirus

• Middle East respiratory syndrome coronavirus

• Moraxella catarrhalis

• Mumps

• Mycobacterium avium-intracellulare

• Mycobacterium haemophilum

• Mycobacterium kansasii

• Mycoplasma pneumoniae

• Neisseria meningitidis

• New Haven coronavirus

• Nocardia

• Paragonimiasis

• Parainfluenza

• Pneumocystis jiroveci

• Pneumonic plague

• Proteus

• Pseudomonas aeruginosa

• Chlamydophila psittaci (psittacosis)

• Rat-bite fever

• Respiratory syncytial virus (RSV)

• Rhinovirus

• Rhodococcus equi

• Streptococcus pneumoniae (pneumococcus)

• SARS

• Serratia

• Severe acute respiratory syndrome

• Sporotrichosis

• Staphylococcus aureus

• Streptococcus agalactiae (group B streptococcus)

• Streptococcus pyogenes (group A streptococcus)

• Streptococcus pneumoniae

• Swine flu

• Toxocariasis

• Toxoplasma gondii

• Trichosporon

• Trypanosomiasis

• Typhus

• Varicella-zoster virus

• Yersinia enterocolitica

• Yersinia pestis

• Zygomycosis

Causes in Aphabetical Order

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Microbiological Etiology

Typical Bacteria	Atypical Bacteria	Viruses
Streptococcus pneumoniae Haemophilus influenzae Staphylococcus aureus Escherichia coli Klebsiella pneumoniae Pseudomonas aeruginosa	Mycoplasma pneumoniae Chlamydophila pneumoniae Legionella pneumophila	Influenza Parainfluenza Respiratory syncytial virus (RSV) Metapneumovirus Adenovirus

Most Common Etiologies for Community-Acquired Pneumonia

Outpatient	Inpatient (non-ICU)	Inpatient (ICU)
Streptococcus pneumoniae Mycoplasma pneumoniae Haemophilus influenzae Chlamydophila pneumoniae Influenza A and B, adenovirus, respiratory syncytial virus, parainfluenza	Streptococcus pneumoniae Mycoplasma pneumoniae Haemophilus influenzae Legionella Aspiration Influenza A and B, adenovirus, respiratory syncytial virus, parainfluenza Yersinia enterocolitica	Streptococcus pneumoniae Staphylococcus aureus Legionella Gram-negative bacilli Haemophilus influenzae Acinetobacter baumannii

Causes by Pathogen

Click here for bacterial pneumonia

Click here for viral pneumonia

Click here for fungal pneumonia

Causes by Age

Common Causes by Age Group

Pathogen	Neonates	Children	Adults
Bacteria	Listeria monocytogenes Mycobacterium tuberculosis	Mycoplasma pneumoniae Chlamydophila pneumoniae Legionella pneumophila Chlamydia trachomatis	Typical Bacteria Streptococcus pneumoniae Haemophilus influenzae Escherichia coli Klebsiella pneumoniae Pseudomonas aeruginosa Atypical Bacteria Mycoplasma pneumoniae Chlamydophila pneumoniae Legionella pneumophila
Virus	Herpes simplex virus Adenovirus Mumps Enterovirus	Respiratory syncytial virus (RSV) Metapneumovirus Adenovirus Parainfluenza Influenza Rhinovirus	Influenza Parainfluenza Respiratory syncytial virus (RSV) Metapneumovirus Adenovirus

• Newborn infants, children, and adults are at risk for different spectrums of disease causing microorganisms.
• In addition, adults with chronic illnesses, who live in certain parts of the world, who reside in nursing homes, who have recently been treated with antibiotics, or who are alcoholics are at risk for unique infections.

Infants

Source of Infection

• Aerosol
• Aspiration of amniotic fluid
• Blood-borne infection across the placenta

Newborn

• Most common cause is Streptococcus agalactiae (Group B Streptococcus)
• GBS causes at least 50% of cases of CAP in the first week of life.
• Other bacterial causes in the newborn period include Listeria monocytogenes and tuberculosis
• Viral causes like herpes simplex virus (most common) adenovirus, mumps, and enterovirus

Children

• For the most part, children older than one month are at risk for the same microorganisms as adults.
• However, children less than five years are much less likely to have pneumonia caused by mycoplasma pneumoniae, chlamydophila pneumoniae, or legionella pneumophila.
• In contrast, older children and teenagers are more likely to acquire mycoplasma pneumoniae and chlamydophila pneumoniae than adults.
• A unique cause of CAP in this group is chlamydia trachomatis, which is acquired during birth but does not cause pneumonia until 2-4 weeks later.
• Common viruses include respiratory syncytial virus (RSV), metapneumovirus, adenovirus, parainfluenza, influenza, and rhinovirus.
• RSV in particular is a common source of illness and hospitalization.
• Fungi and parasites are not typically encountered in otherwise healthy infants, though maternally-derived syphilis can be a cause of CAP in this age group.

Adults

Viruses

• Viruses cause 20% of CAP cases.
• Common viruses are influenza, parainfluenza, respiratory syncytial virus, metapneumovirus, and adenovirus.
• Less common viruses include chicken pox, SARS, avian flu, and hantavirus.

Atypical Organisms

• Mycoplasma pneumoniae, chlamydophila pneumoniae, and legionella pneumophila are often grouped as atypical pneumonia. Community acquired pneumonia caused by these agents present insidiously, with a non-productive cough and prominent extra-pulmonary complaints, such as myalgias and diarrhea (lack the typical pneumonia symptoms of fever, cough, and sputum).
• Mycoplasma pneumonia is often called is "walking pneumonia." It is transmitted via respiratory droplets and is common among healthy individuals in close contact with one another, such as dormitories or military barracks.
• Atypical organisms are more difficult to grow, respond to different antibiotics, and were discovered more recently than the typical bacteria discovered in the early twentieth century.

Streptococcus Pneumoniae

• Streptococcus pneumoniae is the most common cause of community acquired pneumonia.
• Aspiration pneumonia is most commonly caused by anaerobic organisms.
• Prior to the development of antibiotics and vaccination, it was a leading cause of death.
• Traditionally, it was highly sensitive to penicillin, but during the 1970s resistance to multiple antibiotics began to develop.
• Current strains of "drug resistant Streptococcus pneumoniae" or DRSP are common, accounting for twenty percent of all streptococcus pneumoniae infections.
• Adults with risk factors for DRSP including being older than 65, having exposure to children in day care, alcoholism, other severe underlying disease, or recent treatment with antibiotics should initially be treated with antibiotics effective against DRSP.

Hemophilus Influenzae

• Another common bacterial cause of CAP.
• First discovered in 1892, it was initially believed to be the cause of influenza because it commonly causes CAP in people who have suffered recent lung damage from viral pneumonia.

Enteric Gram Negative Bacteria

• Includes E.coli and K.pneumoniae
• Adults with risk factors for infection, which include living in a nursing home, serious heart and lung disease, and recent antibiotic use should initially be treated with antibiotics effective against enteric Gram negative bacteria.

Pseudomonas Aeruginosa

• Uncommon cause of CAP, but it is a particularly difficult bacteria to treat.
• Individuals who are malnourished, have bronchiectasis, are on corticosteroids, or have recently had strong antibiotics for a week or more, should initially be treated with antibiotics effective against Pseudomonas aeruginosa.

Special Situations

• Coccidioides are common in southwestern US.
• Anaerobic infection is common in alcoholics. Pneumococcal pneumonia remains the most common cause of CAP in alcoholics too.
• Psittacosis (due to Chlamydophila psittaci) should be considered in the patient with exposure to birds or bird droppings.
• Anaerobes are common in patients with poor dental hygiene, and a suspected large volume of aspiration.
• Streptococcus pneumoniae, H.influenzae, moraxella catarrhalis, and legionella species are the common causes of community acquired pneumonia in chronic obstructive pulmonary disorders and smokers.
• S. pneumoniae, gram negative bacilli, H.influenzae, staphylococcus aureus, anaerobes, and chlamydia pneumoniae are more common in nursing home residents.
• S. pneumoniae, H.influenzae, and mycobacterium tuberculosis are common pathogens in early stages of HIV, whereas, P.jiroveci, histoplasma, and cryptococcus are commonly seen in late stages HIV.
• In patients with structural lung disease such as bronchiectasis and cystic fibrosis, pseudomonas aeruginosa, Burkholderia cepacia (pseudomonas), and staphylococcus aureus are the common pathogens involved.

Aspiration Pneumonia Causes

• Incompetent swallowing mechanism, such as in neurological disease (a common cause being strokes) or while a person is intoxicated.
• Iatrogenic causes such as general anaesthesia for an operation. Patients are therefore instructed to be nil per os (NPO) for at least four hours before surgery.
• Whether aspiration pneumonia represents a true bacterial infection or a chemical inflammatory process remains the subject of significant controversy.

Drug Side Effect

• Blinatumomab
• Belimumab
• Boceprevir
• Ceritinib
• Dornase Alfa
• Enfuvirtide
• ethanolamine oleate
• Felbamate
• Iloperidone
• interferon alfacon-1
• Pegylated interferon alfa-2b