I thought I had the flu, but after I wasn't getting any better, I went to the doctor. After my exam and an X-ray, I was diagnosed as having walking pneumonia. I was prescribed antibiotics and other medications. Exactly what is "walking pneumonia," and why is it called that?

W.B.

"Walking pneumonia" is an imprecise term meaning that the patient has a lung infection but can walk into and out of the hospital, rather than requiring admission for hospital care. It generally means that the patient is not very ill.

There are many causes of mild pneumonia, and I'll describe a few of the types that you may have had. First, maybe you did have the flu. Influenza is a viral illness characterized by the abrupt onset of fever, headache, body aches and cough. There may be some mild nasal symptoms such as runny nose. While most people have an uneventful recovery, some get complications such as pneumonia. The most common type of pneumonia after the flu is a bacterial "superinfection" (a bacterial infection "on top of" the patient's existing flu infection). Influenza makes a person much more susceptible to bacterial infections of the lung. In some people, this pneumonia may be severe and even fatal, but others will have a milder illness that could be termed "walking pneumonia."

Another form of pneumonia can be directly caused by the influenza virus. This disease is often overlooked because the patient is not much sicker than with typical influenza. An X-ray will show infection of the lung, but the patient may not feel very ill. This could also be the "walking pneumonia" that you experienced. (A second type of pneumonia caused by the flu virus is called "primary influenza pneumonia." This is a very severe illness characterized by involvement of most if not all of both lungs. This form of pneumonia is often fatal, even if patients are placed on breathing machines, and is certainly not what you had.)

A host of other viruses can cause mild pneumonia. They include adenovirus, parainfluenza virus, respiratory syncytial virus and others. These infections cannot be diagnosed accurately without sophisticated testing. In addition, they generally run their course on their own and are not usually treated directly, at least in adults.

A few other organisms cause "walking pneumonia." Probably the most common is Mycoplasma pneumonia. The classic symptoms are fever (usually not very high), dry cough and sore throat. Sometimes the patient feels short of breath, but most patients do not require admission to the hospital. A more recently described organism, Chlamydia pneumoniae, can cause a very similar illness. Both Mycoplasma and Chlamydia are usually seen in young adults. Neither can be diagnosed without special testing.

There are a few other causes of "walking pneumonia," but from what you told me, there is no way to diagnose your illness specifically. As to your treatment, it seems correct. Since it is difficult to easily tell in a doctor's office whether you had a viral or a bacterial pneumonia, most doctors would give antibiotics. If you were sicker and were admitted into the hospital, I would attempt to make a more specific diagnosis. But in your case, I would have probably done exactly what your physician did.