The good news is that I know a guaranteed way to boost your immune system for free. The bad news is that it requires you to stop eating some of your favorite foods. In 1973 the American Journal of Clinical Nutrition published a study that showed how consumption of sugar correlated directly with a diminished function of the immune system, or more accurately, neutrophilic phagocytosis. Neutrophils are major players in our immune system and sugar basically causes them to not function properly, so, essentially, sugar makes your immune system lazy. High fructose corn syrup was introduced in the 70s, but was not mentioned in the study; however, the study does state that fructose had the most significant impact, albeit by a very close margin.
What this means is that anyone who is prone to catching seasonal illness or who is being exposed to contagious pathogens should not eat sugar.
For the sake of staying well, avoid sugar if:
A) You are prone to seasonal colds
B) You are going to be or have been exposed to contagions pathogens, i.e. taking a flight, working or living with children, etc.
There are certainly other reasons why most people should avoid sugar but this is one thing too many people are unaware of. Orange juice is loaded with sugar. Most cough drops are almost entirely sugar. There are benefits to both when you have a cold, but the sugar may negate them, so it is typically better to take a vitamin C supplement and taking prescribed herbs if you do get sick. Eat right, exercise, and get enough sleep. Your body should take care of the rest.
Am J Clin Nutr November 1973 vol. 26 no. 11 1180-1184