Recently a friend and I have had a disagreement over whether the body's ability to fight off a virus or bacterium is diminished if you are outside in inclement weather. For many years our mothers would tell us to bundle up so we wouldn't "get pneumonia." Is there any truth to this new notion that your chances of getting sick are not increased even if you don't dress appropriately for the weather?

Bob F.

What a great reason to have a disagreement with a friend! Unfortunately, I do not believe that anyone has a definitive answer to your question, certainly not definitive enough to declare a winner in your argument. There are few if any good medical research studies attempting to show whether cold weather itself predisposes people to infections such as pneumonia. However, one study that considered cold temperatures as a predisposing factor to infection with rhinoviruses (the group of viruses that cause the common cold) showed no relationship.

I believe that being out in bad weather is not likely to increase your chances of getting sick. There are, however, some theoretical ways in which inclement weather could increase the risk. Some viruses survive and replicate better in cold weather. Also, there is a possibility that very cold weather could interrupt some of the lungs' natural defenses against infection.

I think, however, that there is a far simpler explanation for why many people think that bad weather causes pneumonia. Influenza is a very common infection, affecting more than 10 percent of all people each season. Flu season occurs in winter months, apparently mainly because people crowd indoors to get out of the bad weather. Cold weather may also affect virus survival. When there is a flu epidemic, as many as 40 percent of the population gets sick, and many develop pneumonia. Because there is a relatively high likelihood of getting sick in the winter months, and because most of these people are likely to have been exposed to bad weather, many will become ill soon after being outside in bad weather, simply by chance alone.

We also are biased and remember vividly the times we get sick after being in a terrible snowstorm or rainstorm. This is why anecdotal accounts are not considered good scientific evidence. People remember best the events that suit their favorite theory. Why would anyone remember all the times when being out in bad weather did not result in an illness?

The same problem occurs in the lay press when a case history is recounted in which a person describes how a remedy cured a supposedly incurable illness. This remedy may have been tried thousands of times in other people without success, but when it seems to work once, it makes the news. Unless the evidence is studied in a controlled, scientific manner, these anecdotal accounts do not prove much at all.