A 2009 study published in Archives of Internal Medicine found that those with low vitamin D levels were 40 percent more likely to report respiratory infections. In those with asthma, low vitamin D levels increased likelihood of respiratory infection five-fold.
"There's several randomized controlled trials demonstrating that vitamin D supplementation, especially in children, reduces the risk of influenza," says Dr. Greg Plotnikoff, medical director of the Institute for Health and Healing at Abbott Northwestern Hospital in Minneapolis.
And vitamin D deficiency can be more prevalent than expected. In his study of 10,000 employees in the Atlanta Health Care system in Minneapolis, Plotnikoff found that 63 percent had low levels of vitamin D in their blood. Supplementing with 2000 IU of vitamin D a day would have been enough to bring 99 percent of those employees up to proper vitamin D levels.
Plotnikoff also notes that the 400 IUs that current serves as the recommended guideline for daily vitamin D consumption is an outdated, one-size-fits-all recommendation established in the 1940s by a study on children. In his expert opinion, 2000 IU a day during flu season would be the best way to reduce risk of cold and flu.
Another well-known stress buster, sex has also been shown to have immune-boosting effects when had regularly. A study done at Wilkes University found that those who had sex one to two times a week had elevated levels of IgA, while those who abstained or those who had sex more frequently had significantly lower concentrations of the protein in their system.
Researchers were puzzled by the fact that having sex three or more times a week did not have the same positive effect on the immune system, but thought the evidence supported moderately frequent sexual activity as a way to boost levels of IgA by as much as 30 percent.
Non-sexual touch also confers healthful benefits for a dampened immune system. Though getting a massage is often thought of as a good treatment for sore muscles or a bad back, researcher from the University of Miami School of Medicine finds that regular massage treatments boost immunity as well.
Massage increases the activity level and number of the body's natural "killer cells," which fight off pathogens, and decreases the amount of cortisol (the stress hormone) in the blood.
While some of the above tips may be particularly fun ways to ward off illness this flu season, doctors emphasize that these methods should only complement the standard flu-fighting efforts of vaccination, hand washing and dodging contamination.
"When it comes to preventing colds and boosting the immune system, it is really important to know what is proven to be effective, what has no proof but isn't dangerous and what has no proof and might be even harmful," says Dr. Richard Besser, ABC News Chief Health and Medical editor. "Colds are caused by viruses and most are spread by contaminated hands. So ... hand wash, hand wash, hand wash."
As for complementary methods, "as long as you don't give up on keeping your hands clean and staying home when you are sick, go for it," he adds.
Given that seasonal flu vaccination is now recommended for everyone over the age of six months, Schaffner also urges patients to "run, don't walk to the local pharmacy for vaccination" if they haven't already.
"Real flu is just now taking off around the country," he says. "February is usually the flu's biggest month, so I would remind people that it's not too late to get vaccinated against influenza."