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.

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 contraction that keeps substances out of the lungs, is not working properly.
Infections can spread through the bloodstream from other organs to the lungs.

However, in normal situations, the airways protect the lungs from substances that can cause infection.

The nose filters out large particles.
If smaller particles pass through, sensors along the airway prompt you to cough or sneeze. This forces many particles back out of the body.
Tiny particles that reach the small tubes in the lungs (bronchioles) are trapped in a thick, sticky substance called mucus. The mucus and particles are pushed up and out of the lungs by tiny hair-like cells called cilia that beat like a drum. This action is called the "mucociliary escalator."

Click the icon to see an image of respiratory cilia.

If bacteria or other infectious organisms manage to avoid the airway's defenses, the body's immune system will attack them. Large white blood cells called macrophages destroy the foreign particles.

Click the icon to see an image of a macrophage.

The above-mentioned defense systems normally keep the lung healthy. If these defenses are weakened or damaged, however, bacteria, viruses, fungi, and parasites can easily infect the lung, producing pneumonia.

The Lungs

The lungs are two spongy organs in the chest surrounded by a thin, moist membrane called the pleura. Each lung is composed of smooth, shiny lobes; the right lung has three lobes and the left has two. Approximately 90% of the lung is filled with air. Only 10% is solid tissue. There are several parts to each lung.

When a person takes a breath (inhales), the air travels from the trachea (windpipe) into the lung through the main bronchus, which branches into tiny flexible tubes called bronchi.

The bronchi divide, like the branches of a tree, smaller airways called bronchioles.

The bronchioles lead to a group of microscopic sacs called alveoli, which look like a cluster of grapes. Each healthy adult lung contains millions of tiny alveoli. (Note: The singular of alveoli is alveolus.)

Click the icon to see an image of the lungs.

Each alveolus has a thin membrane that allows oxygen and carbon dioxide to pass in and out of the capillaries, the smallest of the blood vessels. When you take a deep breath, it unfolds and expands. Fresh oxygen moves into the capillaries and carbon dioxide passes out the capillaries, then out of the body through the lungs.

Blood vessels carry the oxygen-rich blood to the heart, where it is pumped through the body.

Review Date: 3/28/2006
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital