You walk outside without a jacket.
Someone immediately tells you:
“You’re going to catch a cold.”
Most of us have heard some version of it.
And technically, being cold does not create a cold or flu. You still need to encounter a virus to become infected.
But the idea that winter weather has nothing to do with why we get sick more often isn’t quite right either.
Respiratory infections such as influenza, rhinovirus and several other viruses follow clear seasonal patterns, particularly in temperate climates. Research suggests winter creates a combination of conditions that can favour respiratory infections: cooler temperatures, lower humidity, more time spent indoors with other people, and potentially weaker antiviral defenses in the nasal passages.¹
So does cold weather “weaken your immune system”?
Not exactly.
But it may make the environment in your airways a little more favourable to viruses.
Let’s break down what the research actually says.
The common cold is caused by viruses.
Rhinoviruses are among the most frequent causes, although coronaviruses, adenoviruses and other respiratory viruses can produce similar symptoms.
That means going outside with wet hair or forgetting your scarf cannot spontaneously cause an infection.
You still need exposure to a virus.
But this is where the old advice becomes more interesting.
Cold weather may affect what happens after that exposure.
Respiratory virus seasonality appears to result from an interaction between the virus, the environment, human behaviour, and our own immune defenses.¹
So instead of asking, “Does being cold cause illness?” a better question might be:
Does winter create conditions that make respiratory viruses more likely to spread or successfully infect us?
Research suggests the answer can be yes.
Before a respiratory virus reaches deeper parts of the body, it often encounters the cells lining your nose.
And those cells aren’t passive.
The nasal epithelium is an important component of the body’s innate immune system, helping recognize pathogens and mount antiviral responses.
Recent research has identified another fascinating layer to this defense.
When nasal cells detect viral signals, they can release large numbers of tiny particles called extracellular vesicles. These vesicles can interact with viruses and participate in the local antiviral response.²
Researchers examining human nasal tissue found that cooler temperatures impaired this response. Exposure to cold reduced both the number of antiviral extracellular vesicles released and aspects of their ability to neutralize viruses.²
In other words, when the temperature inside the nose drops, one of its local antiviral defense systems may become less effective.
That doesn’t mean your entire immune system suddenly becomes “weak.”
It’s a local effect at one of the first places respiratory viruses encounter your body.
The viruses themselves may benefit from the temperature difference too.
Rhinoviruses, which are responsible for many common colds, replicate particularly well at temperatures similar to those found inside the nasal cavity.
Laboratory research found that rhinovirus replication was more efficient at approximately 33°C, a typical nasal airway temperature, than at the warmer 37°C temperature found deeper in the body.³
Part of the explanation appears to involve the immune response.
In airway cells studied at warmer temperatures, researchers observed stronger interferon-mediated antiviral responses than they did at cooler temperatures.³
Interferons are signaling proteins that help cells recognize and respond to viral infection.
More recent work in human nasal cells has similarly found that common cold-associated respiratory viruses can replicate preferentially at nasal airway temperatures and that temperature influences local interferon responses.⁴
So temperature doesn’t just affect how comfortable we feel.
It can influence the interaction between a respiratory virus and the cells trying to stop it.
If cold temperatures were the entire explanation, respiratory virus seasonality would be simple.
It isn’t.
Humidity appears to matter too.
Cold winter air generally contains less moisture. When that air is heated indoors, relative humidity can fall even further.
That may influence respiratory infections in several ways.
Low humidity can affect how long certain viruses survive outside the body and how efficiently they travel through the air. Research on influenza has repeatedly linked cool, dry conditions with more favourable conditions for viral survival and transmission.⁵˒⁶
In an influential experimental analysis, lower absolute humidity was strongly associated with greater influenza virus survival and transmission.⁵
The exact relationship between temperature, relative humidity and absolute humidity is complicated, and researchers continue to debate which measurement best explains influenza seasonality.⁶
But the larger point is fairly consistent:
The physical environment can influence how respiratory viruses behave.
Humidity doesn’t only matter to the virus.
It may also matter to you.
The respiratory tract has a protective system called mucociliary clearance.
Mucus traps particles and microorganisms, while tiny hair-like structures called cilia help move that mucus out of the airways.
Dry air can interfere with that process.
Researchers have proposed that prolonged exposure to cold, dry air can contribute to airway drying and impair mucociliary function, potentially reducing one of the respiratory tract’s mechanical defenses against infection.⁷
So winter may create a double challenge:
Viruses may survive or transmit more effectively under certain environmental conditions, while the respiratory tract may simultaneously become somewhat less effective at stopping them.
Winter changes human behaviour too.
When it’s dark, cold and snowy outside, we spend more time indoors.
Often with closed windows.
Often with other people.
That creates more opportunities for respiratory viruses to move between hosts.
The seasonality of respiratory infections cannot be explained by weather alone. Reviews of the evidence consistently point to a combination of environmental factors and human behaviour.¹
Indoor air quality matters as well.
A 2024 systematic review of 178 studies examining respiratory viral transmission in indoor environments found that ventilation and other indoor-air factors play important roles in viral transmission risk.⁸
Think about the environments we move into every winter:
Offices.
Classrooms.
Public transit.
Holiday gatherings.
Gyms.
Restaurants.
Homes with closed windows.
The weather doesn’t infect you.
But winter can create many more opportunities to encounter the viruses that do.
This is where wording matters.
Saying “cold weather weakens your immune system” makes it sound like stepping outside suddenly shuts down your body’s immune defenses.
That’s not what the evidence shows.
A more accurate way to describe it is:
**Cold and dry conditions may alter certain local immune defenses in the respiratory tract and create environmental conditions that favour transmission of some viruses.**¹˒²
Your immune system is still functioning.
But viruses are also responding to the environment.
And so are your airways.
Winter respiratory illness is not one single wave caused by one virus.
Different viruses have different seasonal patterns.
Influenza typically peaks during colder months in temperate regions. Rhinoviruses often rise in autumn and spring. Respiratory syncytial virus and seasonal coronaviruses also demonstrate seasonal patterns.¹
Climate can influence those patterns, but the relationship varies around the world.
For example, influenza tends to peak during cold, dry conditions in temperate climates. In tropical and subtropical regions, outbreaks may instead align with rainy or humid seasons.⁹
That tells us something important.
It’s probably not “cold” itself that explains everything.
Respiratory virus seasonality emerges from a much more complex interaction between:
temperature + humidity + viruses + host immunity + human behaviour.
This is where interesting research can turn into social media overreach.
Studies showing that cooler nasal temperatures can affect antiviral immunity do not mean scientists have proven that wearing a scarf over your nose will prevent you from getting sick.
The mechanistic research helps explain why respiratory infections may become more common in colder conditions.
It doesn’t automatically establish a specific prevention strategy.
That’s an important distinction whenever new immune research starts circulating online.
A biological mechanism can be real without immediately translating into a clinical recommendation.
You don’t need to obsess over keeping your immune system “boosted.”
Your immune system isn’t a battery that needs to stay at 100%.
Instead, focus on reducing exposure where possible and supporting the basic physiological systems involved in immune function and recovery.
That means:
Getting adequate sleep.
Eating enough and maintaining a varied diet.
Staying physically active.
Managing chronic stress where possible.
Keeping up with recommended vaccinations.
Washing your hands when appropriate.
Improving ventilation when spending time indoors.
And staying home when you’re sick so you’re less likely to pass an infection to someone else.
None of these strategies creates an impenetrable immune system.
That’s not the goal.
The goal is to reduce your risk where you realistically can.
There’s another reason the phrase “boost your immune system” can be misleading.
You can do everything “right” and still catch a respiratory virus.
Exposure matters.
The characteristics of the virus matter.
Your existing immunity matters.
The amount of virus you’re exposed to may matter.
Environmental conditions matter.
And chance matters.
Getting a cold doesn’t automatically mean you failed to eat the right foods, take the right supplement, exercise enough, or manage your stress perfectly.
Your immune system’s job isn’t necessarily to prevent every pathogen from ever entering your body.
It’s to recognize threats and respond to them.
So, does cold weather really weaken your immune system?
Not in the simple way we’ve been told.
Cold weather doesn’t cause a viral infection on its own, and stepping outside without a jacket does not suddenly switch off your immune system.
But the winter environment can still matter.
Cooler temperatures may reduce some of the antiviral defenses in the nasal passages.²˒³
Cold, dry conditions can affect respiratory virus survival and transmission.⁵˒⁶
Dry air may interfere with normal respiratory defenses.⁷
And spending more time indoors increases opportunities for viruses to spread.¹˒⁸
So your grandmother wasn’t entirely wrong.
She just needed a little more immunology.
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