Pneumonia and bronchitis are "lower respiratory diseases" since they affect your lungs or the airways leading into the lungs. "Upper respiratory diseases" affect your nose, throat, and sinuses.

Pneumonia

Pneumonia is a serious infection of the lungs. The air sacs (alveoli) in the lungs fill with pus or other liquid. This blocks the normal exchange of oxygen in the lungs, resulting in less oxygen getting into the blood. Without oxygen, the cells in your body do not work properly. Without proper treatment, this lack of oxygen, combined with the spread of infection throughout your body, can cause death.

Until 1936, pneumonia was the leading cause of death in the United States. Since then, the use of antibiotic drugs has brought the disease under control. Between 1979 and 1996, pneumonia and influenza (flu) together were the fifth leading cause of death.

Bronchitis

Bronchitis affects the air passages (bronchi) that carry air to the lungs from the main airway (the trachea). Inflammation from infection or irritation damages the cells that line the airways. These cells normally contain tiny hairs (cilia) that trap and remove foreign particles you breathe in every day. When the cilia stop working, the air passages become clogged with debris, which increase the irritation even more. As a response, the airways start to produce a thick substance (mucus) which causes a characteristic type of cough. As the irritation continues, the airways become more vulnerable to further infection and tissue damage.

Bronchitis may be acute or chronic. Brief bouts of acute bronchitis (lasting about 10 days) often occur following a severe cold or flu. However, bronchitis may start on its own as well, without any prior infection. Chronic bronchitis lasts 3 months or more, and symptoms can return year after year. Chronic bronchitis can be life-threatening. The condition may result from a series of acute bronchitis attacks, or it may develop gradually because of heavy smoking or inhaling polluted air.

Nearly three quarters of all cases of acute bronchitis are caused by viruses while many cases of chronic bronchitis are due to repeated exposures of irritants such as smoke. This makes antibiotic therapy unlikely to be of help since these drugs can only affect bacterial illnesses and cannot eradicate viral illnesses or irritants. Additionally, a type of asthma which produces little wheezing, but considerable amounts of cough, is often misdiagnosed as acute bronchitis.

Avoiding irritants, maintaining good nutrition, and being patient are the most effective therapies for most cases of bronchitis. Some cases or viral bronchitis can take 8 to 12 weeks to resolve. In prolonged cases of bronchitis, a trial of antibiotic therapy or even asthma inhalers may be helpful. Severe chronic bronchitis may cause dilation of the bronchi (called bronchiectasis) which makes them more susceptible to severe and, sometimes, drug-resistant bacterial infections. Avoidance of irritants such as cigarette smoke becomes even more important in these cases.

Causes

Major causes of pneumonia and bronchitis include:

• Viruses
• Chemical pollutants or dust
• Bacteria

Other major causes of pneumonia include:

• Mycoplasmas: the smallest free-living organisms that cause disease.

Other causes:

• Pneumocystis carinii: a fungus which causes pneumonia in perhaps 4 of 5 people with AIDS
• Inhaled food, liquid, gases, dust, or fungi
• Spirochete: causes Rocky Mountain spotted fever, Q fever, typhus, all of which affect the lungs.
• Tuberculosis pneumonia: a very serious and extremely dangerous disease, if it is not treated early.