Anthony Komaroff, M.D., is professor of medicine and editor-in-chief of Harvard Health Publications at Harvard Medical School. Dr. Komaroff also is senior physician and was formerly director of the Division of General Medicine at Brigham and Women's Hospital. Dr. Komaroff has served on various advisory committees to the federal government, and is an elected Fellow of the American Association for the Advancement of Science.

Question:

Is pneumonia contagious?

Answer:

Boy, you wouldn't think a question that was just three words long would require such a complicated answer. But it does.

Pneumonia means inflammation in the tiny air sacs (called alveoli) that fill the lung. It is in the alveoli that oxygen from the air passes into our body, and that body waste material (the gas called carbon dioxide) passes out and into the air. So when the alveoli get inflamed and can't perform those two vital functions well, people can get pretty sick.

Many kinds of pneumonia are caused by infectious germs of different types, such as bacteria, viruses and funguses. Many, but not all, of these germs are contagious. When a person with pneumonia caused by one of these germs coughs into the air you breathe, you can inhale the germs into your lungs and catch pneumonia from them.

Some of the germs that cause pneumonia, like the virus that causes influenza (the flu), are highly contagious. Other germs are harder to catch from another sick person, but easier to catch in other ways. For example, Legionnaire's disease is a pneumonia caused by a bacterium that sometimes contaminates air conditioning systems or water heating systems. You catch it from being exposed to air or water handled by those systems, not from people with the disease. Some kinds of pneumonia, except on rare occasions, affect only people whose immune systems are seriously weakened. One example is pneumocystis pneumonia. It affects patients with AIDS.

Some types of pneumonia are not caused by germs — at least as far as we know — and are therefore not contagious.

Here are some tips to protect yourself against getting, or giving, pneumonia:

If you are sneezing or coughing, cover your nose and mouth with a tissue and then throw it away. Don't leave it around for others to pick up. If you don't have a tissue, don't sneeze or cough into your hand; use your upper sleeve. Afterward, wash your hands thoroughly with soapy, warm water for about 20 seconds. If water is not available, use an alcohol-based hand gel.

Don't be a strap hanger. On public transportation, loop your arm around a pole or strap rather than holding it with your hand. The same goes for holding the moving side rails on an escalator.

Stay home when you're sick. This simple step can make a big difference in reducing the number of people exposed. If you do go to work or school or come in contact with people, use good hygiene practices. Excuse yourself from shaking hands or kissing others in greeting. A simple, "Sorry, I've got a bug," will get you off the hook.

If people around you are sneezing or coughing, try to avoid getting too close to them, or shaking hands with them (or kissing them).

Talk to your doctor about getting an annual flu shot and a Pneumovax shot (protection against a common cause of bacterial pneumonia).