- Not everyone who is exposed to cold and flu viruses will become sick.
- Susceptibility plays a critical role in determining whether you contract a cold or the flu.
- High stress levels, sleep deprivation and less than optimal levels of vitamin D are important factors influencing susceptibility to such viruses.
There are many theories circulating about the flu, and given the vast amount of information available, it is challenging to separate facts from fiction.1 Nevertheless, data is available to show what makes us vulnerable to these infections and how we can remain healthy when the weather turns cold.1
Although there may be some truth to the claim that bugs are getting nastier and that our crowded environments are responsible for the spread of viruses, it is important to understand that contracting the flu is not a random event.1 Think about it—if exposure to the virus were the only factor determining whether you catch the flu, then everyone would get sick every time they were exposed to the virus.1 2 In a situation where several people in a confined area are exposed to a cold or flu virus, only some become infected.
The implication here is that susceptibility, and not exposure, plays a critical role in whether you become sick.1 2 While those who are diagnosed with medical conditions may be more susceptible to the flu, this article focuses on susceptibility among those who are generally healthy.
The Stress Connection
It has long been recognized that stress has the ability to alter the immune system’s function. You may have noticed that when you are under stress, you are likely to catch a cold or the flu. There is significant evidence to show that stress is one of the biggest risk factors that puts otherwise healthy people at risk for catching infections.3
In a study conducted to investigate the association between psychological stress and susceptibility to common rhinoviruses ( i.e. the virus that causes the common cold), 394 healthy individuals were asked to complete a questionnaire assessing their stress levels.4 The people were then exposed to five respiratory viruses after which they were quarantined and observed for the development of symptoms.4 Twenty-seven percent of the low stress group displayed clinical symptoms, in comparison to 47% of those who were categorized with high stress levels.4
In another study examining the relationship between the duration of stress (long and short-term stress) and severity of illness, 276 persons completed life stressor interviews and psychological questionnaire.5 The people were then inoculated with common cold viruses and monitored for any symptoms. Blood levels of natural-killer (NK) cells (the immune system cells that fight infections), were measured in everyone.5 Those who were stressed for more than a month had lower NK-cell activity and were twice as likely to contract colds.5
The Sleep Connection
The health benefits of adequate sleep are grossly underestimated. Sleep deprivation is common among many people in the United States. A 2013 Gallup poll showed that 40% of Americans get less than seven hours of sleep per night.6 7 There is extensive research suggesting that lack of adequate sleep can compromise our immune system’s ability to fight off colds and the flu.7
According to a recent study published in the journal Sleep, adults who sleep less than six hours per night are four times at a higher risk of contracting a cold than those who sleep at least seven hours, when exposed to the virus.8 Furthermore, getting less than five hours of sleep per night is found to result in a 4.5 times higher risk of catching a cold.8
Aric Prather, the study’s lead investigator and assistant professor of psychiatry at the University of California in San Francisco, states:
We know that sleep plays an important role in regulating the immune system. When we don’t sleep enough, our internal environment shifts to make us less effective at fighting off a virus.”9
This is really the first convincing evidence that objectively verified sleep is associated with susceptibility to the common cold, which is a huge deal for the sleep research community.”9
Although sleep itself will not cure a cold or flu, it will significantly help protect you from catching the virus to begin with.9
The Vitamin D Connection
Similar to stress and sleep deprivation, vitamin D deficiency influences your susceptibility to the common cold and flu. Vitamin D is known to play a key role in the immune system’s ability to respond to pathogens.10 Vitamin D is known as a “miracle nutrient” for your immune system and influences approximately 2,000 to 3,000 of 25,000 genes in the human body.10 It assists the body to produce over 200 antimicrobial peptides, which are necessary to combat infectious diseases.10
According to the U.S. Centers for Disease and Control Prevention (CDC), it is estimated that adults have an average of 2-3 colds per year, and children have even more.11 The prevalence of the common cold and flu, particularly during the winter months, may be attributed to vitamin D deficiency among many people in the United States.10 Research findings support that contracting colds and the flu may be due to an underlying vitamin D deficiency.12 13 In a large study involving 19,000 Americans, those individuals with the lowest vitamin D levels reported catching many more colds or cases of the flu than those who maintained optimal levels of vitamin D.12
The research clearly highlights that inadequate levels of vitamin D has the ability to alter your immune response, leaving you susceptible to contracting respiratory diseases.14
Given all we know about how our body’s immune system works, it makes sense to protect yourself this winter season by managing your stress levels, sleeping more, and ensuring that you are receiving the right amount of vitamin D, in addition to eating nutritious foods and exercising regularly.
1 Thomas P. Colds and Flu: Keeping Well in the Winter Season. What Doctors Don’t Tell You. 12 (8).
2 Mercola J. The First Thing To Do When A Cold or Flu Strikes. Mercola.com Nov.13, 2011.
3 Klein TW. Stress and Infections. Journal of the Florida Medical Association 1993; 80(6): 409-11.
4 Cohen S, Tyrell DA, Smith AP. Psychological Stress and Susceptibility to the Common Cold. The New England Journal of Medicine 1991; 325: 606-12.
5 Cohen S, Frank E, Doyle WJ, Skoner DP, Rabin BS, Gwaltney JM Jr. Types of Stressors That Increase Susceptibility To The Common Cold in Healthy Adults. Health Psychology 1998; 17(3): 214-23.
6 Jones J. In U.S, 40% Get Less Than Recommended Amount of Sleep. Gallup.com Dec. 19, 2013.
7 Mercola J. Lack of Sleep Can Quadruple Your Risk of Catching the Cold. Mercola.com Sept. 15, 2015.
8 Prather A, Janicki-Deverts D, Hall M, Cohen S. Behaviorally Assessed Sleep and Susceptibility to the Common Cold. Sleep 2015 38(9): 1353-59.
9 Oaklander M. Lack of Sleep Dramatically Raises Your Risk for Getting Sick. TIME Aug. 31, 2015.
10 Mercola J. Why Are Stress Makes It Harder to Kick the Common Cold. Mercola.com Apr. 25, 2012.
11 Centers for Disease Control and Prevention. Common Colds: Protect Yourself and Others. National Center for Immunization and Respiratory Diseases, Division of Viral Diseases Oct. 6, 2015.
12 Cannell JJ, Vieth R, Umhau JC, Holick MF, Grant WB, Madronich S, Garland CF, Giovannucci E. Epidemic Influenza and Vitamin D. Epidemiology and Infection 2006; 134(6): 1129-40.
13 Cannell JJ, Zasloff M, Garland C, Scragg R, Giovannucci E. On the Epidemiology of Influenza. 2008; 5(29).
14 Mercola J. Can Vitamin D Cure the Common Cold? Mercola.com Mar. 21, 2009.