I was in excellent health, and one morning I had flulike symptoms. Eight days later, after a chest X-ray, I was diagnosed as having viral pneumonia. Is it common for this type of pneumonia to come on so rapidly? After diagnosis I was given an antibiotic. How does an antibiotic help a viral problem?

G.Y.

Pneumonia (infection of the lung) can be caused by bacteria, viruses, fungi and even parasites. When most people think of pneumonia, they think of bacterial pneumonia, but it is not always possible to distinguish viral from bacterial pneumonia based on the patient's signs and symptoms. Both illnesses can be severe, and even fatal, depending on the specific pathogen (disease-causing organism) involved. If there is significant sputum (saliva mixed with mucus) production, pleural effusion (fluid around the lung) and a dense, localized infiltrate (shadow) on X-ray, then bacterial pneumonia is usually present. If none of these are seen, then it could be either viral or bacterial.

In essentially all cases of pneumonia, a chest X-ray confirms the diagnosis. You were diagnosed with pneumonia after eight days of illness, but you may have had pneumonia even earlier. Pneumonia of any kind often develops a few days after a flu-like illness. When viruses cause pneumonia, the illness starts like any viral respiratory infection. The patient simply keeps getting worse. Bacterial pneumonia also commonly follows an initial viral infection. In fact, the viral infection predisposes to bacterial pneumonia, by damaging some of the lung's defenses against infection. One important clue to this diagnosis is deterioration after initial improvement.

Eight days is definitely not too quick for pneumonia. You can have severe pneumonia on the first day of illness. I think it would be impossible for a physician to be certain that you had viral pneumonia. Therefore, antibiotics are almost always given to a patient with respiratory complaints and an X-ray suggestive of pneumonia because not giving antibiotics is risky. In the very rare case that viral pneumonia is definitively diagnosed, then antibiotics are not helpful and should not be given.