Why Do We Catch More Colds and Flu in Winter? The Main Causes

People catch more colds and flu in winter mainly because respiratory viruses spread more easily then: people spend more time indoors in close contact, cold and dry air helps flu viruses survive and travel, and breathing cold air weakens some of the nose’s first-line defenses. Cold weather itself does not cause a cold or the flu. A virus has to reach you, and winter conditions give viruses more chances to do that.

In the United States, flu activity usually starts to climb in October and most often peaks between December and February, according to the Centers for Disease Control and Prevention. Cold viruses circulate all year, with busy periods in fall, winter and spring.

Does being cold make you sick?

Being cold does not make you sick by itself. Colds are caused by viruses, most often rhinoviruses, and the flu is caused by influenza viruses. Going outside with wet hair or without a coat cannot create an infection if no virus is present.

The old advice to bundle up still has some logic behind it. Cooling the body and the inside of the nose may make it a little easier for a virus you have already picked up to take hold. So the chill can tip the balance, but the virus has to come first.

Why do viruses spread more easily indoors in winter?

Viruses spread more easily in winter because people gather in closed rooms with less fresh air. Schools are in session, holidays bring families and travelers together, and windows stay shut to keep the heat in.

Respiratory viruses travel in droplets and smaller particles that people breathe out when they talk, cough or sneeze. In a crowded room with poor ventilation, those particles build up in the air instead of blowing away. Some viruses also land on hands, doorknobs and phones, and people pick them up and touch their eyes, nose or mouth.

Children are a large part of the picture. They catch many colds each year, share them at school and daycare, and bring them home to the rest of the family.

How does dry winter air help the flu?

Cold outdoor air holds very little moisture, and heating that air indoors makes it even drier. Influenza viruses survive longer and stay airborne more easily in cold, dry conditions than in warm, humid ones.

In humid air, the tiny droplets that carry a virus absorb water, grow heavier and fall to the ground sooner. In dry air they shrink, stay light and can float for longer, which gives the next person more time to breathe them in. This is one reason flu seasons in temperate countries line up so closely with the coldest, driest months, while in many tropical places flu tends to follow rainy seasons instead.

Dry air also dries out the lining of the nose and throat, which leads to the next reason.

Does cold air weaken the immune system?

Cold air weakens some local defenses in the nose and airways, although it does not shut down the immune system as a whole. The inside of the nose is the body’s first barrier against airborne viruses, and it works best when it is warm and moist.

• Mucus traps particles, and tiny hairs called cilia sweep that mucus toward the throat to be swallowed. In cold, dry air the mucus thickens and the cilia move more slowly.
• Breathing cold air cools the inside of the nose, and immune cells there respond less strongly at lower temperatures.
• Blood vessels in the nose narrow in the cold, which can slow the arrival of immune cells.

Sharp cold snaps, such as the Arctic outbreaks described in our guide to how the polar vortex sends cold air south, push people indoors and dry the air even further.

Does less sunlight in winter play a part?

Less sunlight may play a part, though scientists still debate how large it is. The skin makes vitamin D when exposed to sunlight, and in winter, especially at northern latitudes, many people make much less. Vitamin D helps regulate the immune system, and low levels are common in winter.

Research has not settled whether low vitamin D directly raises the risk of colds and flu, so it is best seen as a possible contributing factor. Anyone wondering about their own vitamin D levels should ask a doctor rather than start supplements on their own.

Shorter days also change daily routines. People spend more time inside, sleep patterns can shift, and the holiday season often brings travel and late nights.

Which winter viruses cause the most illness?

Several different viruses cause winter respiratory illness, and they overlap in time:

• Rhinoviruses are the most common cause of the common cold and circulate most of the year.
• Influenza viruses cause the flu, which usually comes on suddenly with fever, aches and tiredness and can be serious for older adults, young children, pregnant people and those with long-term health conditions.
• Respiratory syncytial virus (RSV) causes cold-like illness in most people but can be dangerous for infants and older adults.
• Seasonal coronaviruses cause ordinary colds, and the virus behind COVID-19 also tends to rise in winter.

Because the symptoms overlap, it is often hard to tell which virus is responsible without a test.

How can you lower your risk in winter?

The steps with the strongest support are the ones public health agencies repeat every year:

1. Get a yearly flu vaccine. The CDC recommends one for nearly everyone 6 months and older, ideally by the end of October.
2. Wash your hands with soap and water for at least 20 seconds, especially before eating and after being in public places, or use a hand sanitizer with at least 60% alcohol.
3. Avoid touching your eyes, nose and mouth with unwashed hands.
4. Stay home when you are sick, and cover coughs and sneezes with a tissue or your elbow.
5. Let fresh air in where you can, for example by opening a window for a few minutes or running a ventilation fan.
6. Keep indoor humidity moderate if your air is very dry. Many health and housing guides suggest roughly 30% to 50%, and a clean humidifier can help; humidifiers need regular cleaning so they do not grow mold.

Seek medical care if breathing becomes difficult, a fever is very high or lasts several days, or symptoms improve and then return worse. People at higher risk of flu complications should contact a doctor early, because antiviral medicines work best when started soon after symptoms begin. MedlinePlus, from the US National Library of Medicine, has plain-language pages on colds, flu and when to see a doctor.

Quick answers

Can you catch a cold from going outside with wet hair?

No. A cold requires a virus. Getting chilled may make it slightly easier for a virus you already carry to cause symptoms, but wet hair alone cannot give you a cold.

Why is flu season in the summer in Australia?

Seasons are reversed in the Southern Hemisphere, so Australia’s winter runs from June to August. Its flu season peaks in those months for the same reasons: crowding indoors and cold, dry air.

Is it a cold or the flu?

A cold usually starts gradually with a runny nose and sore throat. The flu tends to hit suddenly with fever, body aches, chills and exhaustion. A test is the only reliable way to tell them apart.

When is it too late to get a flu shot?

Flu viruses often circulate into spring, so the CDC advises getting vaccinated even later in the season if you missed the fall window.