Pneumonia can be caused by microorganisms, irritants and unknown causes. When pneumonias are grouped this way, infectious causes are the most common type.
The symptoms of infectious pneumonia are caused by the invasion of the lungs by microorganisms and by the immune system's response to the infection. Although more than one hundred strains of microorganism can cause pneumonia, only a few are responsible for most cases. The most common causes of pneumonia are viruses and bacteria. Less common causes of infectious pneumonia are fungi and parasites.
Viruses invade cells in order to reproduce. Typically, a virus reaches the lungs when airborne droplets are inhaled through the mouth and nose. Once in the lungs, the virus invades the cells lining the airways and alveoli. This invasion often leads to cell death, either when the virus directly kills the cells, or through a type of cell controlled self-destruction called apoptosis. When the immune system responds to the viral infection, even more lung damage occurs. White blood cells, mainly lymphocytes, activate certain chemical cytokines which allow fluid to leak into the alveoli. This combination of cell destruction and fluid-filled alveoli interrupts the normal transportation of oxygen into the bloodstream.
As well as damaging the lungs, many viruses affect other organs and thus disrupt many body functions. Viruses can also make the body more susceptible to bacterial infections; for which reason bacterial pneumonia often complicates viral pneumonia.
Viral pneumonia is commonly caused by viruses such as influenza virus, respiratory syncytial virus (RSV), adenovirus, and metapneumovirus. Herpes simplex virus is a rare cause of pneumonia except in newborns. People with weakened immune systems are also at risk of pneumonia caused by cytomegalovirus (CMV).
Bacteria typically enter the lung when airborne droplets are inhaled, but can also reach the lung through the bloodstream when there is an infection in another part of the body. Many bacteria live in parts of the upper respiratory tract, such as the nose, mouth and sinuses, and can easily be inhaled into the alveoli. Once inside, bacteria may invade the spaces between cells and between alveoli through connecting pores. This invasion triggers the immune system to send neutrophils, a type of defensive white blood cell, to the lungs. The neutrophils engulf and kill the offending organisms, and also release cytokines, causing a general activation of the immune system. This leads to the fever, chills, and fatigue common in bacterial and fungal pneumonia. The neutrophils, bacteria, and fluid from surrounding blood vessels fill the alveoli and interrupt normal oxygen transportation.
Bacteria often travel from an infected lung into the bloodstream, causing serious or even fatal illness such as septic shock, with low blood pressure and damage to multiple parts of the body including the brain, kidneys, and heart. Bacteria can also travel to the area between the lungs and the chest wall (the pleural cavity) causing a complication called an empyema.
The most common causes of bacterial pneumonia are ''Streptococcus pneumoniae'' and "atypical" bacteria. Atypical bacteria are parasitic bacteria that live intracellular or do not have a cell wall. Moreover they cause generally less severe pneumonia, thus atypical symptoms, and respond to different antibiotics than other bacteria.
The types of Gram-positive bacteria that cause pneumonia can be found in the nose or mouth of many healthy people. ''Streptococcus pneumoniae'', often called "pneumococcus", is the most common bacterial cause of pneumonia in all age groups except newborn infants. Pneumococcus kills approximately one million children annually, mostly in developing countries. Another important Gram-positive cause of pneumonia is ''Staphylococcus aureus'', with ''Streptococcus agalactiae'' being an important cause of pneumonia in newborn babies. Gram-negative bacteria cause pneumonia less frequently than gram-positive bacteria. Some of the gram-negative bacteria that cause pneumonia include ''Haemophilus influenzae'', ''Klebsiella pneumoniae'', ''Escherichia coli'', ''Pseudomonas aeruginosa'' and ''Moraxella catarrhalis''. These bacteria often live in the stomach or intestines and may enter the lungs if vomit is inhaled. "Atypical" bacteria which cause pneumonia include ''Chlamydophila pneumoniae'', ''Mycoplasma pneumoniae'', and ''Legionella pneumophila''.
Fungal pneumonia is uncommon, but it may occur in individuals with immune system problems due to AIDS, immunosuppresive 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 in the southwestern United States.
A variety of parasites can affect the lungs. These parasites typically enter the body through the skin or by being swallowed. Once inside, they travel to the lungs, usually through the blood. There, as in other cases 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 pneumonias (IIP) are a class of diffuse lung diseases. In some types of IIP, e.g. some types of usual interstitial pneumonia, the cause, indeed, is unknown or idiopathic. In some types of IIP the cause of the pneumonia is known, e.g. desquamative interstitial pneumonia is caused by smoking, and the name is a misnomer.
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