Pneumonia Cause

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Pneumonia is an acute respiratory infection in which the aveolar sacs of the lungs become inflamed, often filling with fluid or pus.

The infection causes coughing (sometimes producing bloody or green-yellow mucous), chest pain, fever, chills, nausea, and headaches. Coughing, sneezing, and close personal contact can spread the disease from person to person.

Pneumonia is caused by a variety of organisms, including several types of bacteria, viruses, fungi, or mycoplasma, which is a bacteria-like organism. In total, there are over 30 different causes of pneumonia infection.

Bacterial Infection

The leading cause of pneumonia in adults is by infection with Streptococcus pneumonia, also referred to as pneumococcus. Steptococcus pneumonia is a gram-positive bacteria that is a member of the Streptococcus family.

In healthy individuals, S. pneumonia normally resides in the throat and nasal cavity and does not cause disease. However, the bacterium can spread to the lungs and cause infection in people who have suppressed or under-developed immune systems, such as infants, the elderly, or people who are suffering from a chronic disease, have experienced a recent cold or flu infection, or who have recently undergone chemotherapy. S. pneumonia infection of the lungs causes fever, cough, difficult or painful breathing, and mental disorientation or delirium.

This bacterium can also infect other regions of the body and cause meningitis, ear/sinus infections, and bacteremia (bacterial blood infection).

A vaccine is available to protect people against S. pneumonia infection, and it is strongly recommended for patients belonging to high-risk groups.

Atypical pneumonia, also called walking pneumonia, is a more mild form of pneumonia that is caused by infection with other forms of bacteria including Legionella pneumophila, Mycoplasma pneumoia, and Chlamydophila pneumonia.

Patients with atypical pneumonia typically experience more mild symptoms, hence the name “walking pneumonia”, that can include chills, cough, shortness of breath, chest pain while coughing, or mild fever.

Mycoplasma pneumonia is typically found in younger adults (under age 40), and although symptoms are typically mild, they can include sore throat, ear pain, and rash (in addition to coughing, chills, and fever).

Patients infected with Legionella pneumophila, known as Legionnaire disease, are typically older (over age 50) and may be smokers or have weakened immune systems. In addition to the traditional symptoms, patients with Legionnaire disease may also experience headaches and muscle aches. If left untreated, Legionnaire disease can be life-threatening. Bacterial pneumonia is generally treated with antibiotics.

Viruses and Other Causes

Viruses are the most common cause of pneumonia in children. Common cold viruses, influenza virus, respiratory syncytial virus, and rhinovirus all result in upper respiratory infections that can spread to the lungs and cause pneumonia.

Generally, viral pneumonia is less severe and shorter in duration than bacterial pneumonia. However, some forms of viral pneumonia, especially that caused by influenza, can become quite severe, especially in pregnant women or in patients who have pre-existing heart or lung disease. Symptoms of viral pneumonia include cough, difficulty breathing, wheezing, and flu-like symptoms.

Viral pneumonia is often complicated by a subsequent infection with bacterial pneumonia. Viral pneumonia is not treated with antibiotics unless a bacterial infection is also present.

Fungi can also cause pneumonia. Air-borne fungal spores can be inhaled, causing them to lodge deep in the lungs and trigger an immune response, creating an infection.

Fungal spores can also be found in soil or animal droppings. Fungal pneumonia is most common in immunocompromised patients, such as those with HIV infection, or those patients with a weaken immune system due to chronic disease. Patients with fungal pneumonia can be treated with anti-fungal agents.

Further Reading

Last Updated: Aug 23, 2018

Susan Chow

Written by

Susan Chow

Susan holds a Ph.D in cell and molecular biology from Dartmouth College in the United States and is also a certified editor in the life sciences (ELS). She worked in a diabetes research lab for many years before becoming a medical and scientific writer. Susan loves to write about all aspects of science and medicine but is particularly passionate about sharing advances in cancer therapies. Outside of work, Susan enjoys reading, spending time at the lake, and watching her sons play sports.


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