A “potentially historic” blizzard is barreling down on us here in New England, and is poised to drop up to two feet of snow on Boston. All of the schools in the area preemptively closed, our public transit system is shutting down at 3:30pm, and trying to buy groceries last night was bedlam. The snow is just now beginning to fall. Winter in New England can be rough, especially for a California-raised boy like me. It’s not just because of the snow and cold, it’s also the influenza and common colds.

Source - Flickr user "Placbo"

Source – Flickr user “Placbo”

The fact that the rate of some infections can vary by season is not controversial – actually, it was noted by Hippocrates over 2000 years ago – but in many cases, we still don’t really know why. In some cases, it’s obvious – the bacterium that causes Lyme disease is transmitted by deer ticks, so infections peak in the late spring and early summer when the ticks are active and people are going on hikes. But the winter-peak seasonality of infectious diseases like influenza is harder to pin down.

The first potential explanation is the temperature. After all, there’s a disease called the cold for pete’s sake. We know that cold temperatures can cause physical stress, and that stress can suppress the immune system. Another potential source of immunosuppression is vitamin D deficiency. Most of our Vitamin D comes from a process called “photoconversion” – ultraviolet radiation from sunlight causes a chemical reaction that produces vitD from 7-dehydrocholesterol. In the winter, especially at northern latitudes, the sun isn’t out as long, you’re not outside as much and more of your skin is covered up when you are outside. A number of recent studies have suggested many people are vitD deficient during the winter, and that this might have negative impacts on the immune system.

Both of these explanations seem straightforward enough, but neither explain why some pathogens are seasonal and some aren’t. If it was just immunosuppression, we would expect all pathogens to be more prevalent or at least more severe in the winter, but this is not the case.

My favorite explanation is the human behavioral one. During the winter, people stay inside more, so they’re in closer contact with more people. Infectious disease that spread via direct contact or because of sneezing etc like influenza and the common cold are precisely the pathogens that we’d expect to peak because of this behavior. Unfortunately, this explanation also isn’t complete. Why woud there be seasonality in Florida or southern California for this reason? You could argue that people in northern latitudes are the main driver, and act as vectors for more southern populations (actually, I would argue that that’s the case).

One of the least satisfying (but still totally valid) potential explanations is that the seasonality occurred by chance. Influenza infections occur in cycles of infection -> immunity -> mutation, where after one round, just about everyone that was susceptible is either immune or dead, and the virus must mutate to become infectious again. It’s possible that this annual cycle just happened to peak in the winter. Because of the interactions between different pathogens, this would only have to happen to a single virus in order to get the whole thing going. In other words, the rhinovirus responsible for the common cold* might just be riding the coat-tails of influenza to hit people that are already knocked down by infection. There are issues with this hypothesis too. In the southern hemisphere, flu infection is also seasonal and also falls during the winter, which is off-set by 6 months from the northern hemisphere winter, but this alone does not rule out the “by chance” hypothesis.

There are other possible explanations as well, but all of them are very difficult to test. At best, we have plausible explanations with supporting correlations, but that’s not enough to make a definitive claim. Even more problematic, it’s possible that all of these explanations are true to some extent, and cooperate to give us the seasonality that we see.

In any case, my fellow East Coasters, bundle up, be safe, and try not to sneeze on anyone.


Most of this information was learned at some point during my schooling, but for a source, check out this review

*There are actually several different viruses that cause the symptoms associated with “the cold,” but the most common is Rhinovirus.


  1. [...] article “Snow, cold, influenza and colds-Temperature and infectious Disease” is about seasonal pathogens seem to be seasonal while others are not. The article speculates that [...]

  2. #2 Alex R.
    February 12, 2013

    I never thought that the increase in the the common cold in the winter would of been due to just the plain fact that that disease goes through it’s own cycle. That is a great point that I never thought about. However, all the other points you brought up about Vitamin D, and being in close contact with other individuals I don’t think can be pushed aside. Great article! Made me think outside the box.

    • #3 Kevin Bonham
      February 12, 2013

      @ Alex – Absolutely. I think the likeliest explanation is that all of these factors play some role, and it’s just a matter of how much they contribute.

  3. #4 Caitlin Thomas
    February 12, 2013

    @ Alex – I, too, think it is important to take all theories into consideration when looking for the best explanation as to why the “common cold” is so common during the winter months. I personally believe that our likelihood to stay indoors, out of the cold, and thus, being out of the sunlight and in closer contact with other bacteria-carrying humans is the most likely explanation as to why the cold follows a cycle; it is because we follow a cycle, as well. The lack of sunlight causing vitamin D deficiency, resulting in a suppressed immune system, leaves us more susceptible to catching the cold because our body will not be able to fight off foreign bodies as readily as it would in warmer months. This, coupled with the increased germ population of being around other people, is what causes us to get sick in the winter.

  4. #5 Nicholas Cubillas
    February 12, 2013

    Feeling as if I am always getting sick throughout the winter months this has always interested me as well. I recently learned about Vitamin D deficiencies in a physiology class and how sunlight has an affect on how the body produces Vitamin D. It may be the fact that it is often cloudy during the winter months, allowing our body not to fully create all the vitD we need and leaving us more susceptible to airborne infections. I liked the point you had about since it is cold out people tend to be inside more in a more confined area with more people around, which also could account for some spread of infections causing the common cold. Good article! Makes me still wonder why I always get sick in the winter but I guess ill still be left in the dark about it for awhile.

  5. #6 Sarah Shockey
    United States
    February 12, 2013

    I think there are many good possibilities for why people get sick in the winter months, and it is definitely worth looking into. In a biology class I took we are discussed how humans have a photochemical process of receiving vitmanins from the sunlight. In most cases the sun is not out as much in the winter and the temperature is decreased so more people stay indoors. When people are indoors more they are less likely to be able to receive vitamin D and other essential vitamins to help fight off sickness. Another key factor is being indoors more and in close contact with other people who are sick making you more likely to catch something contagious.

  6. #7 Cortney Marotto
    United States
    February 12, 2013

    Your points on the many causes of the influenza infections during the winter season made complete sense, in particular, the way in which it has its own cycle that it goes through.
    Like you said, once the infection has built up its’ immunity to the vaccinations, it mutates just a little then again repeats the process. In addition, the body builds up antibodies, the body’s defense, to the virus. When these mutations do arise the following winter, after the virus has mutated, the body’s antibodies may or may not recognize the strain of the virus, if not the antibodies might not be able to fight of the infection. As you mentioned, the virus goes through some sort of mutation each cycle in order to become “infectious again”. This being so true, especially in the recent severe influenza outbreak occurring over the last several months. With this recent outbreak with the flu, many have began to think that there was going to be a pandemic again like the swine flu outbreak in 2009. However, after many hospitalizations and deaths this past season, the flu outbreak seems to be decreasing. It is believed that that the recent severe outbreak in the influenza infection was due to new mutation in the virus. For many, like stated before, when the virus mutated, the antibodies were not able to protect the body from the virus, thus allowing for more than usual number of outbreaks. As being a residential assistant at a very populated university, I have seen many residents that have become sick due to the past flu season. As you mentioned earlier in your article, when winter rolls around, everyone tends to stay indoors more. When living on a floor with about 40 others students, in which everyone shares everything, there are ALWAYS being germs passed between one another. I have seen some of the residents that were sick for close to a week and others for a few days. When residents tends to stay in, due to the cold weather, they become more likely to spread the virus to one another. Additionally, with every student comes varying body types and antibodies produced within their bodies. This causes the virus to affect each of them differently and some more severe then others.
    I know that my reasoning and research on this is very broad and there are many other factors, like you said, that play into the causes of the seasonal influenza.

    I found the following website very interesting in giving in formation about the current flu situation in the world.
    I also used this website to as a source for my reasoning.

  7. #8 Alexis LaNeve
    February 13, 2013

    I have always found this topic very interesting. Why do people get sick so much more in the winter compared to the summer? I agree with what you have said about being indoors more in the winter and receiving less sunlight and being closer to more people so infection can spread easier. There is something that I think can be added to the fact the staying inside causes a vitamin D deficiency. When people are stuck indoors for longer periods of time, as they are during the winter, they tend to be less active. I have learned in my exercise physiology classes that exercising strengthens the immune system. So I think that there is a correlation between being less active and being more prone to sickness. I would like to see studies done on active people vs. inactive people and the occurrences of infections or sickness.

  8. #9 Morgan Drake
    February 13, 2013

    These are very logic points for why people get sick during the colder months. I definatley agree with the whole Vitamen D deficiency causing immunosuppresion. I’ve typically get sick once a month during the winter months. As in the warmer months, I’ve noticed I”m never usually sick probably because I spend a great deal of time in the sun soaking in all the UV rays. I also think that people could be getting sick during the winter too because of physical activity. The less active you are, the weaker your immune system will be. Activeness also hels with energy balance. These are all great topics to discuss on factors affecting the immune system.

  9. #10 Patrick Garcia
    February 13, 2013

    I agree with Alex R., you definitely cannot push aside that a major cause for this seasonality is Vitamin D deficiency and the fact that people are indoors so close in contact with each other. However, it is difficult to think of what else could cause this immunodeficiency in such a seasonal manner, especially in the winter. Generally, viruses travel much slower in colder weather. One would think that spring might be the time when all sickness breaks loose. I mean, you have the change in temperature (effecting sinuses), growth and increase of airborne allergens, and the fact that people are around each other just as much as they were in the winter (if not more because people tend to go out more in warm weather). I know that mucus in the body dries up in the winter which is bad for the body because mucus traps bacteria, but as far as that goes, I still can’t get over the fact that winter has been the season for illness, and we haven’t seen a deviation of this in decades, if not, ever.