Here's the reality your parents may not have revealed when they piled on the sweaters and gloves: Bundling up before you go outside in the winter won't stop you from getting sick.
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"It's not cold weather itself that makes people get the flu," says David Cutler, MD, family medicine physician at Providence Saint John's Health Center in Santa Monica, California. That's true for the common cold, too.
Both these respiratory illnesses are caused by viruses — they spread through the germ-filled droplets sick people emit when they cough, sneeze and talk. You can also get sick by touching a virus-contaminated surface (like someone's hand or the doorknob), per the Mayo Clinic.
But that doesn't mean there isn't any connection at all between the weather outside and getting sick.
"There's no doubt that cold weather is associated with an uptick in cold and flu cases," Michael Huang, MD, national medical director for Marathon Health, an employer-based healthcare provider, tells LIVESTRONG.com.
Not only can colder temperatures create conditions hospitable to viruses, but chilly weather may also make us more susceptible to catching them, Dr. Huang says. Here are some of the factors that may explain why "cold-and-flu season" aligns so well with the fall and wintertime.
Viruses Flourish in Colder Weather
It's commonplace for people to talk about "flu season," which generally occurs from October to May, with a peak in flu cases during February, per the Centers for Disease Control and Prevention (CDC).
And it's not only the influenza virus that has a seasonal aspect. While the rhinovirus (the virus that causes the common cold) is present year-round, it has peaks in the fall and again in March, per a January 2019 study in Scientific Reports, which compared meteorological data from Scotland to viral transmission.
Respiratory syncytial virus, another common respiratory tract virus, is also seen more at colder temperatures, peaking in December, per the same study.
As well as being more prevalent in fall and winter, viruses are more likely to thrive in colder weather, Dr. Huang says. "They reproduce better, they live longer and [they] may stay circulating longer in the air," he says.
The Cold Makes Us Spend More Time Inside
Office workers are all too familiar with the office cold that travels from one cubicle to the next. In the cooped-up confines of the office building, it can feel like there's no escape from the germs that spread from one sneezing coworker to another.
And in the winter, of course, most of our activities and get-togethers take place indoors, since cooler temps make gathering in parks and backyards off-putting.
"Being close together with other people we know transmits viruses that are spread by droplets or aerosol," Dr. Cutler says. "But we don't know that closing the windows and people spending more time inside is the trigger for this being spread more — it probably is, it just hasn't been proven."
Cold, Dry Air Affects Your Nose
Cold temperatures outside, accompanied by high heat indoors, leads to dry air and low humidity.
"There's a lot of evidence that cold, dry air makes you more susceptible to viruses," Dr. Huang says.
For instance, there's an association between rhinovirus infections and a dip in the temperature and humidity in the days preceding the infection, per September 2016 study in Viruses.
A drop in the humidity, along with cold air, can dry out your nasal passageways, which Dr. Huang describes as our body's first line of defense.
"Breathing in cold air can cause narrowing of the blood vessels in your nose. Theoretically that can cause less white blood cells to be distributed into your nasal mucosa — those are the germ-fighting cells," Dr. Huang says.
Plus, viruses may flourish in the dryer air. At lower humidity levels, the flu virus is more infectious, according to a February 2013 study in PLOS One. Keeping the humidity levels at 40 percent (or higher) reduces the infectiousness of an aerosolized flu virus, per the study.
Aim to have the humidity in your home between 30 and 50 percent, advises the Mayo Clinic. You want to aim for this Goldilocks-like sweet spot because too-low levels may increase your susceptibility to viruses, but an overly humid home can lead to the growth of bacteria and mold, which is also a problem for your health.
A hygrometer, which you can purchase online or at a hardware store, can measure your home’s exact humidity levels.
During Winter, You Get Less Vitamin D
"Being markedly deficient in vitamin D does appear to impair your immune function," Dr. Cutler says.
Taking supplements may be helpful at preventing respiratory illnesses, particularly for people with a vitamin D deficiency, per a February 2017 meta-analysis of 25 studies in The BMJ.
So, What Can You Do?
Avoiding colder weather — and the potentially hospitable circumstances it creates for viruses — likely isn't an option. But there is, of course, plenty you can do to prevent exposure to viruses, lessening your likelihood of catching a cold or flu.
The strategies are simple, and likely familiar, Dr. Huang says. Start by getting your flu shot, he recommends. As well, you'll want to eat healthy, stay hydrated, exercise regularly and get enough sleep, he notes.
Finally, keep up with all of your diligent hand-washing and social distancing. "The same methods used to limit the spread of COVID-19 can also work to mitigate the spread of other colds and flu," Dr. Huang says.
- Mayo Clinic: "Common Cold"
- Centers for Disease Control and Prevention: "The Flu Season"
- Scientific Reports: "Association between viral seasonality and meteorological factors"
- Viruses: "A Decrease in Temperature and Humidity Precedes Human Rhinovirus Infections in a Cold Climate"
- PLOS One: "High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs"
- Mayo Clinic: "Humidifiers: Air moisture eases skin, breathing symptoms"
- The BMJ: "Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data"