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3 Reasons Why More Colds and Flu in Winter?

Updated on October 24, 2012

Why do we get more colds and flu during the winter? It is not simply being cold that causes us to get sick. We get sick from colds and flu because we are exposed to viruses which are immune system is not able to fight off.

And it is not because there are any more viruses during the winter than summer. So why do we get sick more often during the winter?

We get more colds and flu during winter because of our immune systems are less optimal due to decreased sun exposure and decreased protection from fresh vegetables and fruits. In addition, viruses are more transmittable from person to person during wintertime when their exterior shell hardens to be gel-like offering them greater ability to be virulent out in the open.

1. Decrease in vitamin D levels

Among the many health benefits, vitamin D boosts our immune system. Many people are deficient in vitamin D to begin with. But they are especially deficient in winter time. This is because vitamin D is generated mainly by the UVB sunlight radiation hitting our skins. We only get very limited amounts from our foods.

Since there is less sunlight during winter, our vitamin D levels decreases, causing our immune system to be in less than optimal shape. This decreases our ability to fight off the viruses that are around us.

The Milwaukee Wisconsin Journal Sentinel reports that "flu outbreaks have occurred around the planet when solar radiation was at its lowest".[1] Also in winter, the sun is at a lower angle in the sky decreasing the about of UVB radiation hits our skin to generate vitamin D.

Researchers have found an association between decreased vitamin D levels and an increase of colds and flu. In a study of 19,000 people, they found that those deficient in vitamin D (less than 10 nanograms per milliliter) were 55% more likely to have had a cold or upper respiratory infection. Whereas those with adequate levels of vitamin D, had 30 nanograms per milliliter of vitamin D.

Read more about the importance of taking vitamin D for both summer and winter.

Dr. Joel Fuhrman writes in Super Immunity that ...

"it has to been suspected for centuries that declining sunlight during winter months could be a factor promoting increases in infections, including influenza" [page 104]

A great video about vitamin D is the one with Dr. Mercola and Dr. Cannell as shown below. Dr. Cannell is the founder of And they talk about the relationship between inadequate vitamin D level and increase colds and flu.

2. Decrease of Fresh Vegetables

The book Super Immunity also stresses that the phytonutrients in vegetables and fruits are an important part of keeping us healthy by maintaining our immune system and tip-top shape. Because most vegetable grows best during summer, the winter months usually means a smaller supply of fresh vegetables.

In particular green and cruciferous vegetables have been shown to help immune cells in the gut and skin function better. The gut and skin are two of the body main defenses against viruses. The nutrient in such vegetables aids in the production and survival intra-ephithelial lymphocytes (IELs), which are a type of white-blood cells that plays a role in wound repair and defense.

Whether you have decreased fresh vegetables depends to some extent on where you live and how far your produce have been shipped from. In some areas, you can get fresh vegetables year-round due to the fact that vegetable are shipped halfway around the world from countries where it is summer to countries where it is currently winter. However, for many people, they are limited by the amount and variety of produce that they can get during winter months.

3. Flu Virus More Infectious During Winter

Both colds and flu are from viruses. Viruses typically do not live very long in open environment without a host. However during wintertime viruses can live longer out in the open air than during summertime. This increases their ability to transfer from one person to another, increasing the infection rates.

The reason why viruses can live longer in the open during winter is because they have a outer covering that hardens and turn gel-like. This hardened exterior helps protect the virus from the elements, making them able to survive outside a host longer. During summer, this covering becomes more liquid-like which confers less protection.[3]

We do not get colds from being cold

The simple state of cold temperature in itself is not responsible for us catching more colds. It is just that viruses are hardier in cold temperatures.

The New England Journal of Medicine has article titled Exposure to Cold Environment and Rhinovirus Common Cold -- Failure to Demonstrate Effect says ...

"this study demonstrated no effect of exposure to cold on host resistance to rhinovirus infection and illness that could account for the commonly held belief that exposure to cold influences or causes common colds"

Oxford Journals reports of another study that found that "Acute chilling of the feet causes the onset of common cold symptoms in around 10% of subjects who are chilled." That's only one in ten people.


This HubPages article was written by author in March 2012 and is only opinion at the time of writing. The author believes in the value of vitamin D supplementation and occasionally take up to 4000 IU of vitamin D split-dose in a day during flu season. This is more than the recommended daily allowance. But author is a health nut and believes that RDA values are too low.

More about colds and flu

Dr. Ben Kim says in his website that it is fine to let the cold and flu run its natural course. Because colds and flu's are caused by viruses rather than bacteria, antibiotics are not appropriate. For healthy individuals, is not necessary to take flu vaccines or other cold medications that suppress symptoms such as coughing and sneezing as long as you stay hydrated and get plenty of rest. But if these medications makes you feel better so that you can get some rest and/or not transmit it to others in public, then that's fine. But it is not necessary to take them "just because" one is sick.

These symptoms are a natural way for the body to clear out toxins. Viruses from colds and flu mainly kills off the body's weakest cells. These are the types of cells most likely to contain excessive waste products and toxins, and are the types of cells you want to get rid of anyways. That way, it can be replaced by new healthy ones.

Dr. Joel Fuhrman has the same sentiment as written in his book Super Immunity.


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