The
cold and flu season is starting to rear its ugly head and we can't seem to get
away from the coughing and sneezing. But why are we prone to get the sniffles
during winter?
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.
Seasonal patterns
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.
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, you might see outbreaks during rainy, humid months, or
relatively consistent levels of flu cases all year round.
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.ject
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?
Source: Medical News Today
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