I have just recovered from illness. For most of last week, I had a chest infection. You may have noticed that infections seem to be quite common in winter. Is it that it’s cold
and wet? This is what I was always told as a kid, and what I get warned about from my mother to this day.
Staring down the barrel at placing Dr in front of my name in under 6 months’ time, I hope I can give you a better answer than just “cold and wet”. Let’s see…
The Infectious Agent
Firstly, when someone gets an infectious disease (e.g. upper and lower respiratory tract infection), this means that they have been in contact with a virus, bacterium, or other micro-organism that is now causing illness. That leads us to the first reason that is probably intuitive: we spend more time indoors during winter. When indoors, we are often closer to others, and often in the absence of appropriate ventilation.
Therefore, this results in a higher probability of transmission from person-to-person, hence why outbreaks often occur in schools.
Sick Environment, Sick
Body?
Along with this, there are factors that contribute to a micro-organism’s stability in the environment. You may recall early on in the pandemic when people were asking all sorts of
questions about sunlight, air temperature, humidity, surface types, and more, with reference to the viability of SARS-CoV-2. Morris et al. (2020) investigated this question and found that the virus survived longer at lower temperatures and the extremes of humidity (very dry or very humid air), suggesting that the dry, cold air of winter
may be conducive of greater environmental stability. This has been replicated beyond just SARS-CoV-2, and it seems like the role of dry air is surprisingly potent, with flu epidemics almost always following drops in air humidity (Shaman et al. 2010).
All we have discussed so far are factors in the environment that impact the micro-organism itself. What about our bodies? Does winter have a weakening effect on us?
Why Our Bodies Weaken
Kudo et al. (2019) sought to answer this question with reference to our resistance to influenza infection. In their study (in mice), they found that low humidity conditions (dry air) resulted in impairments in each of the following:
Mucociliary Clearance
Mucociliary clearance involves surface mucus and little motors in
your airways that propel infectious material up and out of your airways, reducing their potential to replicate and cause illness.
Innate Antiviral
Defense
The innate antiviral defense is the immune system’s frontline defense to acute infection. Without this, protecting the body from new infection becomes increasingly difficult.
Tissue Repair Function
As it suggests, tissue repair mechanisms are essential to repairing and protecting the airways in response to infection. If this is
compromised, we would expect worse symptoms and a longer course of illness.
Along with this, the same research group found that lower temperatures resulted in less immune proteins (Interferons) being
secreted, increasing susceptibility to replication of the rhinovirus, which causes the common cold (Foxman et al. 2015).
Temperature and humidity aside, another gift we get from the sun is Vitamin D. If you were paying attention during the COVID-19 pandemic, you probably heard people yapping about Vitamin D. And for good reason. Vitamin D seems to play an important role in the immune system, both in the innate immune response and cellular immunity, and Vitamin D deficiency is consistently associated with infectious disease outcomes (although often
cause and effect cannot be clearly ascertained) (Hewison 2011). Therefore, the absence of endogenous Vitamin D synthesis from sunlight may be yet another reason why we are more susceptible to infection in winter.
Winter = Illness, or Seasons = Different Illnesses?
Interestingly, many researchers also believe that it’s not necessarily as simple as “winter = more infections”, but rather that some infections may have seasonal patterns that vary by climate. For example, European cohorts demonstrate increased immune cell counts in winter, whereas Gambian cohorts (near-equatorial) exhibited a seasonal peak
during their rainy season from June to October (highest infectious burden for their country) (Dopico et al. 2015). Dr Micaela Martinez of Columbia University also points out that, historically, measles has hit hardest in spring, and polio in summer.
In Summary...
Putting this all together, we can
say that the cold, dry air, combined with people being crammed indoors, are probably the biggest environmental and behavioural contributors to winter illness. Along with this, our bodies may also do a poorer job at immune defense due to the environmental conditions listed above, along with the lack of sunlight. There are likely other contributing factors too, but please remember that there must be a virus, bacterium, or otherwise for you to have an infectious illness. It’s not just because you
had wet hair, as our mammies often tell us.
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