Why do viruses make us sick in the winter?

Viruses exist all year round, but it is only in the winter that they seem to be able to get at us, why is that?

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The old wives tale about cold weather having something to do with it is true. It is not that viruses like the freezing cold, they like a host body temperature of about 32C. The issue is that the cold weather can sometimes bring a person's temperature down from our normal 36C - 37C to about 32C, giving the virus a chance to take hold. Here is how it works.

Humans are famous for the way we can keep our core body temperature at about 36C to 37C all year round. The core body is the bit around our vital organs - heart, lungs, liver, stomach, kidneys, intestines and of course the brain.

However, our surface temperature can drop in cold weather. We have evolved hair on our heads as a way to protect the surface of the skull from getting cold, and we invented clothes for the same reason - without clothes, man couldn't have moved out of the tropics.

But that still leaves our extremities - our hands, feet, ears, mouth and nose.

Viruses can't enter via the hands and feet, and the entrance into the ear is protected by wax, and the inner ear is warmed by arteries close by, keeping the temperature at 37C. The mouth similarly has blood vessels in the throat, tongue and under the tongue, keeping it at core temperature.

That leaves the nose. It is stuck at the front of our faces, in order to take in air, and has only minor blood vessels under the skin. In cold weather the temperature in the nose can drop to 32C, and bingo, the viruses take the opportunity to multiply.

Head colds usually occur when people have left their hair wet in cold temperature, and the body does triage, and diverts all blood to warming the skull (and thus protecting the brain), which leaves the nose even colder.

When a cold takes hold, our bodies respond in several ways. We send white blood cells to attack the viruses directly, we produce copious amounts of mucous in order to sneeze the virus out, and as a last resort we increase our temperature in order to boil the virus to death. When you have a fever, your temperature hits 38C, which is enough to kill any virus (it would kill us too if we kept it up for too long).

The reason both old people and children get colds more easily than everyone else is because the old are no longer efficiently regulating their temperature, and children have immature immune systems, so the white blood cell defence isn't working fast enough.

Keep warm in winter

The way to protect yourself against cold is to keep warm. As long as the temperature in your nose and mouth is at least 36C, viruses should struggle to take hold because it is too warm for them. Heat your house properly, and when outside, wear a scarf protecting your nose and throat from having their surface temperatures drop. And if you see a homeless person, the best thing you can do for them is to buy them a hot drink as that will raise the temperature in their throat and nose.