 Bacteria  Viruses  Fungi  Parasites  Idiopathic

 Bacteria are the most common cause of community acquired pneumonia, with Streptococcus pneumoniae isolated in nearly 50% of cases.  Other commonly isolated bacteria include: Haemophilus influenzae in 20%, Chlamydophila pneumoniae in 13%, Mycoplasma pneumoniae in 3%, Staphylococcus aureus, Moraxella catarrhalis, Legionella pneumophila and gram- negative bacilli.

 Risk factors for infection depend on the organism involved.  Alcoholism is associated with Streptococcus pneumoniae, anaerobic organisms, and Mycobacterium tuberculosis, smoking is associated with Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Legionella pneumophila, exposure to bird with Chlamydia psittaci, farm animals with Coxiella burnetti, aspiration of stomach contents with anaerobes, and cystic fibrosis with Pseudomonas aeruginosa and Staphylococcus aureus.  Streptococcus pneumoniae is more common in the winter.

 In adults, viruses account for approximately a third of pneumonia cases.  Commonly implicated agents include: rhinoviruses,coronaviruses, influenza virus, respiratory syncytial virus (RSV), adenovirus, and parainfluenza.  Herpes simplex virus is a rare cause of pneumonia, except in newborns.  People with weakened immune systems are at increased risk of pneumonia caused by cytomegalovirus (CMV).

 Fungal pneumonia is uncommon, but it may occur in individuals with weakened immune systems due to AIDS, immunosuppressive drugs, or other medical problems.  The pathophysiology of pneumonia caused by fungi is similar to that of bacterial pneumonia.

 Fungal pneumonia is most often caused by Histoplasma capsulatum, blastomyces, Cryptococcus neoformans, Pneumocystis jiroveci, and Coccidioides immitis.  Histoplasmosis is most common in the Mississippi River basin, and coccidioidomycosis is most common in the southwestern United States.

 A variety of parasites can affect the lungs. These parasites typically enter the body through the skin or the mouth. Once inside the body, they travel to the lungs, usually through the blood.  In parasitic pneumonia, as with other kinds of pneumonia, a combination of cellular destruction and immune response causes disruption of oxygen transportation.

 One type of white blood cell, the eosinophil, responds vigorously to parasite infection.  Eosinophils in the lungs can lead to eosinophilic pneumonia, thus complicating the underlying parasitic pneumonia.  The most common parasites causing pneumonia are Toxoplasma gondii, Strongyloides stercoralis, and Ascariasis.

 Idiopathic interstitial pneumonia or noninfectious pneumonia are a class of diffuse lung diseases.  They include: diffuse alveolar damage, organizing pneumonia, nonspecific interstitial pneumonia, lymphocytic interstitial pneumonia, desquamative interstitial pneumonia, respiratory bronchiolitis interstitial lung disease, and usual interstitial pneumonia.