|MadSci Network: Virology|
Like so many questions, the answer to your question could range from simple to very complex. But this is an interesting question, so letís work through it.
First, letís be sure everyone understands what is meant by "the flu". This term originated as a shortened form of the name of the specific viral illness "influenza". Influenza is caused by one of three viruses, Influenza virus types A, B and C. It is a classic disease characterized by headache, fever, cold-like symptoms, aches and pains, and generally feeling like garbage. It can be quite serious, and has been known for a very long time as a cause of "excess winter deaths." The term "flu", though, has become far more widely applied, and in common usage has come to mean ANY illness that causes symptoms similar to influenza (and in fact, some people even refer to the "stomach flu", which has nothing whatsoever to do with influenza!). When you think about it, the symptoms of influenza are really quite non-specific, and in fact many other viruses cause similar symptoms. Doctors recognize this as well, and frequently use the term "flu-like illness". Such a designation often means that no further specific diagnosis will be made, since for most people the only treatment is rest, time, and lots of TLC.
The above is very important in understanding your question. When most people say "the flu", they are really not talking about a single-specific illness!! Rather, it is a whole series of viral infections, many of which overlap with the common cold. Certainly one of these, and the most severe, is influenza. It is the one that you are protected against by the "flu shot".
Now, "flu-like illnesses" can and do occur throughout the year. To realize this, just think of how often you have had the dreaded "summer cold" - many times, if youíre like most people. Yes, there is some degree of seasonality to such illnesses, and yes, they do occur more frequently in non-summer months (although not strictly in the winter). This is almost certainly because of the increased transmission rate that is a consequence of staying in close quarters (see the discussion below). But most non-influenza illnesses are generally not strictly seasonal.
The situation with influenza is different. This disease is strictly seasonal, i.e. it only occurs at specific times of the year, specifically in winter months in temperate climates. In fact, the occurrence of influenza in winter months is probably the very thing that makes "the flu", especially its severe form, more common in winter months. But remember, "the flu" is not a specific term!! Thus, "flu-like illnesses" do occur more commonly in winter months, and some (if not most) of that is due to influenza. But influenza itself has been found to occur essentially exclusively in winter months, while the other viruses are generally not so restricted. I hope thatís clear.
So, why should viruses have a seasonal property?? There are certainly many factors that influence this epidemiology - Iíll mention a few. Some infectious agents that are strictly seasonal are spread by animals and bugs, and so are restricted to the times when those bugs are out (e.g. Lymeís disease and ticks). Other agents that are not strictly seasonal, but that occur more frequently in winter months, are generally explained away by the increased rate of person-to-person transmission that occurs when people stay inside a lot. Basically, most "flu-like illnesses" are spread by close contact, either by aerosols produced by coughing and sneezing or by touching objects in the environment that had been touched by an infectious person. In the winter, we spend more time inside in close contact with other people, and so viral illnesses spread more easily. Bottom line, that is the main reason for the large scale winter phenomenon you address. Might viral factors also play a role? Yes, in particular, viruses need to survive for some time outside of a person. I can imagine that some viruses would not do as well outside of a person in summer months, due to drying and more intense sunlight, etc. A notion that I would like to dispel, though, is that people are more easily infected in winter months. Stated outright, if you come into contact with the same viral source in summer or winter, you will get the same disease. There is no good reason to believe, given the same exposure to virus in winter vs. summer, that you would be more likely to develop the disease in winter. Viruses donít change from winter to summer, and for the most part neither do people. The thing that changes in winter is mostly the rate at which you are exposed to viruses.
With that said, though, I hope some readers are saying "but how does that explain influenza"??? The common cold occurs year Ďround, but with a higher rate of transmission and therefore more cases in winter. This makes sense. But influenza essentially disappears in non-winter months, with potentially widespread epidemics occurring every winter. How can this be due only to increased transmission? Well, it canít. This is an aspect of influenza infection that we simply donít understand fully. I am not an expert on this particular question, but I did a little bit of digging. What I found was a few individuals with theories that are as yet unsubstantiated (and not worth detailing here), and a lot of people who simply seem to ignore the question. Thus, the standard answer remains that influenza patterns are determined by a combination of epidemiological factors, including increased spread within a given area in the winter, as well as a geographic pattern of spread from one world region to another. This will have to do for now, but maybe some day we will understand an aspect of the biology of influenza virus infections that explains their seasonality more fully.
Finally, let me add that there are a lot of other interesting aspects to influenza infection that I havenít touched on here. But this is already quite long, so I guess theyíll have to wait for another question......
Tom Wilson, MD PhD
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