Viral infections that cause the common cold or the flu are a nuisance. According to the Centers for Disease Control and Prevention (CDC), "Common colds are the main reason that children miss school and adults miss work" each year.
While most colds tend to go away by themselves, every year, the flu kills an estimated 250,000 to 500,000 people worldwide.
So, what do scientists know about how plummeting temperatures allow these viruses to spread like wildfire?
Common cold vs. flu
First, we need to distinguish between the common cold and the flu, because the viruses that cause these do not necessarily behave in the same way.
Most of the time, the common cold manifests with a trilogy of symptoms: sore throat, blocked nose, and cough. There are more than 200 viruses that can cause the common cold, but rhinoviruses are by the far the most common culprits.
Interestingly, around a quarter of people infected with a common cold virus are lucky enough not to experience any symptoms at all.
The flu is caused by the influenza virus, of which there are three types: influenza A, influenza B, and influenza C. Common colds and flu share many symptoms, but an infection with influenza often also manifests with a high temperature, aching, and cold sweats or shivers — a good way to tell the two apart.
So, now that we know the difference between the common cold and the flu, let's look at when we tend to fall prey to these viruses.
The CDC monitor flu activity closely. Influenza can occur at any time of year, but most cases follow a relatively predictable seasonal pattern.
The first signs of influenza activity usually start around October, according to the CDC, and often peak at the height of winter. But some years, flu outbreaks can stick around and last until May.
The peak month for flu activity in the seasons spanning 1982–1983 to 2015–2016 was February, followed by December, March, and January.
Other temperate locations across the globe see similar patterns, with cold temperatures and low humidity cited as the prime factors, a 2013 analysis showed. The same can't be said for tropical areas, however.
There, you might see outbreaks during rainy, humid months, or relatively consistent levels of flu cases all year round.
If this seems counterintuitive to you, you are not alone. While influenza data do support such a link, scientists don't fully understand how viruses are able to exert their maximum damage at both low and high temperature and humidity extremes.
There are several theories, however, ranging from the cold affecting how viruses behave and how well our immune system copes with infections, to more time spent in crowded places and less exposure to sunlight.
Cold air affects our first line of defense
Common cold and flu viruses try to gain entry into our bodies through our noses. Luckily, our nasal lining has sophisticated defense mechanisms against these microbial intruders.
Our noses constantly secret mucus. Viruses become trapped in the sticky snot, which is perpetually moved by tiny hairs called cilia that line our nasal passages. We swallow the whole lot and our stomach acids neutralize the microbes.
But cold air cools the nasal passage and slows down mucus clearance. Viruses can now stick around for longer, trying to dig through the snot to break into our body.
Once a virus has penetrated this defense mechanism, the immune system takes control of fighting off the intruder. Phagocytes, which are specialized immune cells, engulf and digest viruses. But cold air has also been linked to a decrease in this activity.
Rhinoviruses actually prefer colder temperatures, making it difficult not to succumb to the common cold once the thermometer plummets. At a normal body temperature, these viruses are more likely to commit cell suicide, or apoptosis, or to encounter enzymes that make short work of them, a 2016 study showed.
Vitamin D and 'spending time indoors' myths
During winter, levels of ultaviolet radiation are much lower than in summer. This has a direct effect on how much vitamin D our bodies can make. Vitamin D has been shown to be involved in making an antimicrobial molecule, which has been shown to limit how well the influenza virus can replicate in laboratory studies.
Consequently, some believe that taking vitamin D supplements during the winter months can keep flu at bay. The findings of a clinical trial published in 2010 showed that school children who took vitamin D-3 daily had a reduced risk of developing influenza A flu infection.
However, the results of a further clinical trial — published earlier this year — found that high levels of vitamin D supplementation had no effect. So, the jury is still out on the effects of vitamin D.
Another reason commonly cited to contribute to cold and flu infections in the fall and winter months is that we spend more time indoors once the weather is less hospitable.
This is thought to have two effects: crowded spaces help viruses to spread from person to person and central heating causes a drop in air humidity, which — as we've already seen — is linked to influenza outbreaks.
However, many of us live our lives in crowded spaces all year round, and in isolation, this theory cannot explain flu rates. Instead, some scientists believe that temperature, humidity, travel, and crowding all contribute to the overall risk of virus transmission.
With many people scheduled to travel over the upcoming Thanksgiving weekend, we predict that the sniffles will be making the rounds in the aftermath of the festivities.
But is there anything that you can do to protect yourself?
How to stave off viruses and treat symptoms
Your chances of catching a cold this winter are very high. In fact, the CDC estimate that adults have two to three colds each year.
The best way to protect yourself is by washing your hands with soap and water frequently, avoiding touching your eyes, nose, and mouth, and staying away from people who are already infected.
These rules also apply to influenza. The CDC recommend a yearly flu shot as the best way of preventing infection.
However, should you fall prey to a winter virus, here are eight home remedies to consider to help ease your symptoms.
You should always contact your doctor if you have a high temperature that doesn't go away, your symptoms persist, get worse, or are unusual, or if you have any chest pain.