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Seasonality of Winter Viruses

Colds Caused by the Cold?

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Updated December 11, 2008

Most of us have experienced the seasonality of colds and flu, with sniffles, sneezes, and coughs occurring much more frequently in the winter months than in the summertime. Why? For one, cold winter climates foster the desire to stay indoors, which often means staying in enclosed, highly populated places where transmission of infectious microbes is optimal. However, for those who live in areas of cold, blistery winters, most have been warned to "bundle up!" or get sick.

But if seasonal illnesses are caused by viruses and other microbes, how does one explain the phenomena of the cold being associated with getting sick more frequently? It turns out there may be some truth to this old wives' tale after all.

Why do people get sick more frequently in the winter?

Researchers from Mount Sinai School of Medicine in NY have found that the Influenza Virus, responsible for causing flu, is actually transmitted more frequently in cold, dry environments than warm, humid ones. In their report, they suggest three possible contributing factors:

  1. The mucous membrane in your nose gets dried out. In normal conditions, the mucous membrane in your nose serves as a protective barrier against infectious microbes, with mucous being a source of antibodies and other antimicrobial molecules. Drying of mucous membranes can hinder their protective effects, as well as cause damage to the mucosal cells, permitting the entry of upper respiratory viruses, such as those responsible for flu and cold, into your body.
  2. The virus itself is more stable in cold, dry climates. Think of it this way: Viruses have a protective "shell", known as the envelope, that is stronger and gel-like in colder temperatures. When the weather warms up, this protein shell is more prone to "melting", and the virus is less able to survive.
  3. Aerosolized droplets of virus remain airborne for longer periods. Respiratory viruses are primarily spread through inhalation of aerosolized droplets after coughing or sneezing. These droplets consist of water plus virus. In areas of high humidity, the droplets take on water and settle out of the air more rapidly than in areas of low humidity, where the droplets remain airborne for a longer period of time. Thus, inhalation of virus-containing airborne droplets is more likely to happen in regions of low humidity and make you sick.

It isn't entirely clear how much any of these factors contribute to the seasonality of winter viruses. Studies are still being conducted to identify the true cause of seasonal illnesses, but it is important to note that there actually exists scientific evidence for this phenomenon.

Is your immune system weaker in cold climates?

The study at Mount Sinai did not show any difference in immune function at high vs. low temperatures and varying levels of humidity, suggesting that the difference in transmission of the flu virus had little to do with changes in the immune system. While there are studies that suggest otherwise, most studies seem to indicate that cold exposure does not suppress the immune system.

Sources:

Lowen AC et al. Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature. PLOS Pathogens. 2007; 3:e151.

Polozov IV, et al. Progressive ordering with decreasing temperature of the phospholipids of influenza virus. Nature Chemical Biology. 2008; 4: 248.

Castellan JW, et al. Cold exposure: human immune responses and intracellular cytokine expression. Med Sci Sports Exerc. 2002;34:2013-20

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