Science

Why do some illnesses come back every winter? The cold-air answer

Rive News Research Desk

Published October 24, 2026 · 6 min read

By Rive News / Research desk | Rive News / Published October 24, 2026 at 9:00 AM PKT

Key takeaways

Cold air itself does not create viruses, but cold dry air helps respiratory viruses survive and spread.
Inhaling cold air constricts blood vessels in the nose, weakening the first line of immune defense.
Flu, RSV, and human coronaviruses reliably peak in winter in temperate regions.

A person coughing in cold winter air, the season when respiratory viruses circulate most

[Winter morning frost at Stowe Gardens, the cold dry air that helps respiratory viruses survive and spread.] Photograph: winter morning frost

Image Credits: Wikimedia Commons

Colds, flu, and other respiratory illnesses are infections caused by viruses that peak in winter in temperate regions. The cold weather does not create these viruses, but it creates conditions where they spread more easily and the body fights them less well.

About 200 different viruses can trigger cold symptoms, including rhinoviruses, coronaviruses, and parainfluenza viruses. In winter, a mix of environment, biology, and behavior brings them out in force.

This article explains why illnesses return each winter, including how cold dry air helps viruses spread, how cold air weakens the body's defenses, why indoor crowding matters, and which illnesses peak and when.

Why doesn't cold air make us sick by itself?

Because colds and flu are caused by viruses, not by temperature. A cold can be caught in summer if the virus is around, and standing in a blizzard cannot conjure a virus from thin air.

What cold air does is make it easier for a virus to gain a foothold once exposure occurs. Researchers describe several distinct mechanisms, and they stack on top of each other during the winter months.

It is a pattern long observed by doctors and parents alike: the waiting rooms fill in December, not because the cold is the illness, but because the cold is the accomplice.

How does cold, dry air help viruses spread?

Winter air in temperate regions is cold and dry, and that combination is unusually good for respiratory viruses.

Laboratory research shows that influenza viruses and some human coronaviruses are more stable and transmissible at about 5°C and 10-40% relative humidity, conditions typical of winter, according to a review published on News-Medical. In contrast, higher temperatures and humidity in summer are associated with reduced viral viability.

Dry air also changes the physics of a cough or sneeze. After exhalation, virus-laden droplets rapidly lose water by evaporation, leaving smaller aerosols that persist in the air for extended periods and travel longer distances. Scientific American describes how the cold even changes the outer membrane of the influenza virus itself, making it more solid and rubbery, which scientists believe makes person-to-person transmission easier.

Surfaces matter too. Cold viruses can survive for up to 2 hours on a given surface, according to a PMC review, so touching a contaminated handle and then your face is enough.

How does cold air weaken the body's defenses?

The nose is the front door for respiratory viruses, and cold air weakens its locks.

Your nose and throat are lined with moist mucous membranes designed to trap germs before they reach the lungs. Dry winter air pulls moisture from these tissues, making them less effective. Mucus becomes thick and sticky instead of smooth and protective, and the tiny cilia hairs that sweep germs out of the airways slow down.

Inhaling cold air also lowers the temperature inside the nose and triggers vasoconstriction, narrowing blood vessels. As Russian outlet Izvestia summarized from medical reporting, this reduces blood flow to the mucous membranes, weakening the local immune response that normally destroys viruses at an early stage.

Less sunlight plays a supporting role. Reduced sun exposure lowers vitamin D synthesis in the skin, and low vitamin D has been associated in observational studies with worse outcomes for some respiratory infections.

Close view of frost and cold air, which helps respiratory viruses survive longer

[An 1892 painting of a winter morning with hoar frost melting, the kind of cold dry air viruses favor.] Photograph: a winter morning painting

Image Credits: Wikimedia Commons

Why does spending more time indoors matter?

Winter drives people inside, and viruses thrive on the proximity.

Crowded spaces with poor ventilation let virus-containing droplets build up in the air, making transmission between people more likely. Schools and workplaces bring large numbers of people into close contact, and the tightly sealed, centrally heated buildings of winter recirculate air that facilitates person-to-person spread, the PMC review notes.

Childhealthy London, a pediatric health information service, puts it plainly: close contact increases the likelihood of viruses spreading between individuals, and good ventilation is harder to maintain in cold weather.

The radius matters. Respiratory viruses generally spread within about 6 feet of an infected person, Scientific American reports. In summer, the same group of people is more often outdoors, spaced out, with moving air diluting the viral cloud.

Which illnesses peak in winter, and when?

Not every virus follows the same calendar.

In temperate regions, wintertime peaks are well described for influenza, RSV (respiratory syncytial virus), and endemic human coronaviruses. RSV typically increases from autumn and continues through winter, and seasonal influenza tends to rise during the same window, according to Childhealthy London.

Rhinoviruses, the most common cause of the common cold, are more of a spring-and-fall story depending on subtype. But the winter lineup of flu plus RSV plus coronaviruses is enough to make the season feel like one long illness.

Americans catch nearly a billion colds each year by some estimates, cited in the PMC review, and a large share of those cluster in the colder months. That clustering is what people mean when they say "cold season," even though winter months are more accurately described as virus-friendly months.

Frequently asked questions

Does going outside without a jacket cause a cold?

No. Colds come from viruses, not from being cold. But the crowded, dry-air indoor environments of winter make it much more likely you will meet one of those viruses.

Why do some people never seem to get sick in winter?

Individual immune history, ventilation at work and home, hand hygiene, and simple luck all play roles. Prior exposure builds partial immunity to specific strains, though new variants arrive every season.

Do humidifiers help prevent winter colds?

Keeping indoor air less dry may help mucous membranes work better and may reduce viral survival, though a humidifier alone cannot stop infection. Aim for moderate indoor humidity and keep the unit clean.

Why are flu seasons worse some years than others?

How well the vaccine matches the circulating strains, how the weather pattern unfolds, and how many people get vaccinated all shift the severity from year to year.

The outlook

Winter illness is not a mystery of weak character or bad luck. It is a predictable collision: viruses that survive best in cold dry air, noses that defend worst in it, and people packed indoors together.

Knowing the mechanisms helps. Ventilate rooms, wash hands, and give your nose warm humid air when you can. This article is for general information only and is not medical advice; talk to a clinician about any symptoms.

Topics: winter illness, flu season, colds, respiratory viruses

Rive News / Research desk covers science explainers, writing from peer-reviewed sources and named medical reporting.

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