We see more and more research establishing that deficiencies in certain nutrients underlie a very wide range of disease conditions. Disease is the result of either nutrient deficiency at the cellular level, or toxicity (by harmful substances) at the cellular level. There is now a huge amount of research available on vitamin D showing that lower levels are associated with many disease states one of which is influenza.

Just by way of example, we'll quote here a research paper and also an article from Scientific American reporting on vitamin D, colds and flu.

Adit A. Ginde, MD, MPH; Jonathan M. Mansbach, MD; Carlos A. Camargo Jr, MD, DrPH. Association Between Serum 25-Hydroxyvitamin D Level and Upper Respiratory Tract Infection in the Third National health and nutrition Examination Survey. Arch Intern Med. 2009;169(4):384-390.

Author Affiliations: Department of Emergency medicine, University of Colorado Denver School of medicine, Aurora (Dr Ginde); and Department of medicine, Children's Hospital Boston (Dr Mansbach), and Department of Emergency medicine, Massachusetts General Hospital (Dr Camargo), Harvard Medical School, Boston.

Background Recent studies suggest a role for vitamin D in innate immunity, including the prevention of respiratory tract infections (RTIs). We hypothesize that serum 25-hydroxyvitamin D (25[OH]D) levels are inversely associated with self-reported recent upper RTI (URTI).

Methods We performed a secondary analysis of the Third National health and nutrition Examination Survey, a probability survey of the US population conducted between 1988 and 1994. We examined the association between 25(OH)D level and recent URTI in 18 883 participants 12 years and older. The analysis adjusted for demographics and clinical factors (season, body mass index, smoking history, asthma, and chronic obstructive pulmonary disease).

Results The median serum 25(OH)D level was 29 ng/mL (to convert to nanomoles per liter, multiply by 2.496) (interquartile range, 21-37 ng/mL), and 19% (95% confidence interval [CI], 18%-20%) of participants reported a recent URTI. Recent URTI was reported by 24% of participants with 25(OH)D levels less than 10 ng/mL, by 20% with levels of 10 to less than 30 ng/mL, and by 17% with levels of 30 ng/mL or more (P < .001). Even after adjusting for demographic and clinical characteristics, lower 25(OH)D levels were independently associated with recent URTI (compared with 25[OH]D levels of e30 ng/mL: odds ratio [OR], 1.36; 95% CI, 1.01-1.84 for <10 ng/mL and 1.24; 1.07-1.43 for 10 to <30 ng/mL). The association between 25(OH)D level and URTI seemed to be stronger in individuals with asthma and chronic obstructive pulmonary disease (OR, 5.67 and 2.26, respectively).

Conclusions Serum 25(OH)D levels are inversely associated with recent URTI. This association may be stronger in those with respiratory tract diseases. Randomized controlled trials are warranted to explore the effects of vitamin D supplementation on RTI.

And the article from Scientific American:

Vitamin D Deficiency Linked to More colds and flu Scientific American (Online) Feb 23, 2009

Is sunshine more than just a home remedy for a cold? New research suggests it may be: In a study that will be published tomorrow, people with low levels of vitamin D - also known as the "sunshine vitamin" - were more likely to catch cold and flu than folks with adequate amounts. The effect of the vitamin was strongest in people with asthma and other lung diseases who are predisposed to respiratory infections.

People with the worst vitamin D deficiency were 36 percent more likely to suffer respiratory infections than those with sufficient levels, according to the research in this week's Archives of Internal medicine. Among asthmatics, those who were vitamin D deficient were five times more likely to get sick than their counterparts with healthy levels. And the risk of respiratory infection was twice as high among vitamin D-deficient patients with chronic obstructive pulmonary disease (COPD) than in lung patients with normal levels of the vitamin.

All this means that healthy adults, who typically get two colds a year, might suffer an extra one if they're vitamin D deficient. For people with asthma or COPD, who get around four or five colds annually, lack of vitamin D might tack on additional infections, but exactly how many isn't known, co-author Adit Ginde tells ScientificAmerican.com.

Ginde, of the University of Colorado Denver School of medicine, and colleagues at Harvard Medical School found the association when they compared vitamin D levels taken from nearly 19,000 participants in the federal government's National health and nutrition Examination Survey (NHANES) with their answers to the question, "In the past few days, have you had a cough, cold or other acute illness?"

Only a study that gave vitamin D supplements to people with low levels and compared their respiratory infection rates with people who had sufficient levels of the vitamin would show a true cause and effect, Ginde says. But, he adds, the results build on previous research suggesting that vitamin D is important to the function of the immune system.

Lab work has shown that lack of vitamin D is associated with weaker production of an antimicrobial peptide called hCAP-18, a protein that works with immune-system cells to kill pathogens. "We think that if you're exposed to a virus [and] you have sufficient vitamin D, those cells will be better equipped to fight off that organism so you don't get an infection," says Ginde, an assistant professor of surgery in his university's department of emergency medicine. In people with vitamin D deficiency, it's possible that "those cells dont work as well so you're more like to get a cold or infection or something more severe."

In people with asthma or COPD, he added, vitamin D deficiency "is a second hit" that may compound their underlying risk of respiratory infection from their disease.

Having 30 nanograms of vitamin D per milliliter of blood is considered optimal. More than half of the people in the study had vitamin D levels below that threshold.

The most recent recommendations from the Institute of medicine (IOM), which are 12 years old, say that Americans should get 200-600 International Units of vitamin D a day. But those recommendations were set based on the vitamin's contribution to bone health, not immunity and overall wellbeing. Proponents of more vitamin D intake, such as Michael Holick, say 1,000-2,000 IUs might be needed. An IOM update to the recommendations is expected in May 2010.

"It's clear that the American population needs more vitamin D overall for its effects on bone health and the growing literature on non-skeletal benefits for general health," says Ginde, who expects participants in an upcoming vitamin D trial will get the amped up levels of 1,000-2,000 IUs that advocates are pushing for. "We're not recommending that everyone go out and take that, but that's the magnitude of change we're talking about."

The content above (which is only a fraction of what is available in the scientific literature) has some interesting implications, one of which is nicely presented in the following paper published in the Virology Journal with regard to patterns of influenza in populations:

John J Cannell(1) , Michael Zasloff(2), Cedric F Garland(3), Robert Scrag(4) and Edward Giovannucci(5). On the epidemiology of influenza. Virology Journal 2008, 5:29.

(1) Department of Psychiatry, Atascadero State Hospital, 10333 El Camino Real, Atascadero, CA 93423, USA. (2) Departments of Surgery and Pediatrics, Georgetown University, Washington, D.C., USA. (3) Department of Family and Preventive medicine, University of California San Diego, La Jolla, CA, USA. (4) Department of Epidemiology and Biostatistics, University of Auckland, Auckland, New Zealand. (5) Departments of nutrition and Epidemiology, Harvard School of Public health, Boston, MA, USA.

The epidemiology of influenza swarms with incongruities, incongruities exhaustively detailed by the late British epidemiologist, Edgar Hope-Simpson. He was the first to propose a parsimonious theory explaining why influenza is, as Gregg said, "seemingly unmindful of traditional infectious disease behavioral patterns." Recent discoveries indicate vitamin D upregulates the endogenous antibiotics of innate immunity and suggest that the incongruities explored by Hope-Simpson may be secondary to the epidemiology of vitamin D deficiency. We identify - and attempt to explain - nine influenza conundrums: (1) Why is influenza both seasonal and ubiquitous and where is the virus between epidemics? (2) Why are the epidemics so explosive? (3) Why do they end so abruptly? (4) What explains the frequent coincidental timing of epidemics in countries of similar latitude? (5) Why is the serial interval obscure? (6) Why is the secondary attack rate so low? (7) Why did epidemics in previous ages spread so rapidly, despite the lack of modern transport? (8) Why does experimental inoculation of seronegative humans fail to cause illness in all the volunteers? (9) Why has influenza mortality of the aged not declined as their vaccination rates increased? We review recent discoveries about vitamin D's effects on innate immunity, human studies attempting sick-to-well transmission, naturalistic reports of human transmission, studies of serial interval, secondary attack rates, and relevant animal studies. We hypothesize that two factors explain the nine conundrums: vitamin D's seasonal and population effects on innate immunity, and the presence of a subpopulation of "good infectors." If true, our revision of Edgar Hope-Simpson's theory has profound implications for the prevention of influenza.

What the authors are saying is that a more primary underlying factor behind influenza is vitamin D deficiency and not what is commonly believed as a "circulating virus", since all the available facts regarding influenza outbreaks and epidemiology cannot be sufficiently explained by "a circulating virus" alone.

And in the conclusion of the above study there occurs:

Compelling epidemiological evidence indicates vitamin D deficiency is the "seasonal stimulus" [22]. Furthermore, recent evidence confirms that lower respiratory tract infections are more frequent, sometimes dramatically so, in those with low 25(OH)D levels [90-92]. Very recently, articles in mainstream medical journals have emphasized the compelling reasons to promptly diagnose and adequately treat vitamin D deficiency, deficiencies that may be the rule, rather than the exception, at least during flu season [40,41]. Regardless of vitamin D's effects on innate immunity, activated vitamin D is a pluripotent pleiotropic seco-steroid with as many mechanisms of action as the 1,000 human genes it regulates [93]. Evidence continues to accumulate of vitamin D's involvement in a breathtaking array of human disease and death. [40,41]

What the authors are concluding is that the "seasonal stimulus" is not actually the "circulating virus" but vitamin D deficiency. In light of all the above, you may want to increase your intake of vitamin D in general (you can take up to 50mcg or 2000IU daily as a safe upper level), and especially in the winter season.

There are other useful articles on vitamin D on this site which you should read, and some of which are listed further below.