The "common cold" is a syndrome that is characterized by signs and symptoms of upper respiratory infection: sneezing, itchy or watery eyes, rhinorrhea, cough, and sinus congestion. Malaise (the feeling of "feeling sick") is reported, but is generally more mild than with the flu. Fever is uncommon with colds. Myalgias and arthralgias (muscle and joint aches) are also reported but are typically more mild than with flu).
The common cold is invariably caused by viruses and more than one family of viruses is capable of producing this syndrome with varying degrees of severity and duration depending on the virus. The best known virus family is the rhinoviridae.
Rhinovirus has at least 99 serotypes. This presents a challenge to the body's immune system as an immune response to one serotype may not provide any protection against other serotypes. It is possible for a person to be consecutively infected with rhinoviridae of different serotypes. The durability of immunity to a rhinovirus strain, once established, is not known. It remains possible that over time, a person can become susceptible to reinfection.
Because human adaptive immunity requires antigen exposure, there is little that someone can do on their own to "boost" their specific immunity against cold viruses. There is no vaccine (so many viruses and serotypes to consider). Boosting non-specific immunity (the acute inflammatory mechanisms) is a strategy that has not been well explored, but this approach is hard to recommend. Non-specific immunity is a two-edged sword with the potential to damage the host's own body in addition to its role as the "first responders" to a pathogen.
Airborne like products. Over the counter products that purport to "boost immunity to colds" are largely clinically untested. Vitamin C has been examined. In a July 2007 study, researchers wanted to discover whether taking 200 milligrams or more of vitamin C daily could reduce the frequency, duration, or severity of a cold. After reviewing 60 years of clinical research, they found that when taken after a cold starts, vitamin C supplements do not make a cold shorter or less severe. When taken daily, vitamin C very slightly shorted cold duration.
Most over the counter immune boosters for colds are vitamin B and C based, which makes them safe and reasonably non-toxic so their use can't be discouraged. Therapeutic doses of water-soluble vitamins usually result in very expensive urine.
Zinc. Recently an analysis of several studies showed that zinc lozenges or syrup reduced the length of a cold by one day, especially when taken within 24 hours of the first signs and symptoms of a cold. Studies also showed that taking zinc regularly might reduce the number of colds each year, the number of missed school days, and the amount of antibiotics required in otherwise healthy children. Zinc can potentially have some side-effects, so you should talk to your doctor before beginning zinc supplementation.
TLDR: Rhinoviridae is very infectious, lives outside the body for hours to days, and is readily transmitted by auto-inoculation from encounters with contaminated surfaces. The best "immune booster" for colds really, truly is frequent hand-washing.
As explained here, exercise is theorized to have a positive effect on the immune system via a few different mechanisms, although the rigorous proof that it does anything at all is still lacking (that's most likely a result of a lack of research in this area). As this source says, you should not overdo it, but then the amount of exercise that will be too much for you depends on your fitness level to begin with. So, the fitter you are the harder you can actually exercise before you pass the break even point where the negative effects start to grow larger than the positive effects.
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