Pneumonia - Introduction

Highlights	Risk Factors	Prevention
Introduction	Diagnosis	Resources
Causes	Treatment	References
Symptoms	Medications
Prognosis	Surgery

Introduction

Pneumonia is an inflammation of the lung that is most often caused by infection with bacteria, viruses, or other organisms. Occasionally, inhaled chemicals that irritate the lungs can cause pneumonia. Healthy people can usually fight off pneumonia infections. However, people who are sick, including those who are recovering from the flu (influenza) or an upper respiratory illness, have weakened immune systems that make it easier for bacteria to grow in their lungs.

When air is inhaled through the nose or mouth, it travels down the trachea to the bronchus, where it first enters the lung. From the bronchus, air goes through the bronchi, into the even smaller bronchioles and lastly into the alveoli.

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Defining Pneumonia by Location in the Lung

Pneumonia may be defined according to its location in the lung:

Lobar Pneumonia occurs in one part, or lobe, of the lung.
Bronchopneumonia tends to be scattered throughout the lung.

Defining Pneumonia by Origin of Infection

Doctors often classify pneumonia based on where you contracted the disease. This also helps predict which organisms are most likely responsible for the illness and, therefore, helps decide treatment.

Community-Acquired Pneumonia (CAP). People with this type of pneumonia contracted the infection outside a hospital setting. It is one of the most common infectious diseases. The disease often follows a viral respiratory infection such as the flu.

One of the most common causes of bacterial CAP is Streptococcus pneumoniae. Other causes include Haemophilus influenzae, mycoplasma, and Chlamydia.

Hospital-Acquired Pneumonia. Hospital-acquired pneumonia is an infection of the lungs contracted during a hospital stay. This type of pneumonia tends to be more serious because hospital patients already have weakened defense mechanisms and the infecting organisms are usually more dangerous than those encountered in the community. Hospital patients are particularly vulnerable to gram-negative bacteria and staphylococci. Hospital-acquired pneumonia is also called nosocomial pneumonia.

Click the icon to see an image of hospital-acquired pneumonia.

Disease Process Leading to Pneumonia

Pneumonia-causing agents reach the lungs through different routes:

In most cases, a person breathes in the infectious organism, which then travels through the airways to the lungs.
Sometimes, the normally harmless bacteria in the mouth can enter the lungs. This usually happens if the body's "gag reflex," an extreme throat contaction that keeps substances out of the lungs, is not working properly.
Infections can spread through the bloodstream from other organs to the lungs.