According to this article, sugar slows down immune system activities. There are certain white blood cells called neutrophils. These things specialize in destroying cold viruses and other unwanted intruders. However the neutrophils become lethargic when you eat sweets.
In one study, researchers had volunteers consume 100 grams of sugar, the equivalent of two cans of soda. Then they took blood samples from the volunteers. They found that neutrophil activity in the volunteers had plummeted by 50 percent after consuming the sugar. Five hours later, neutrophil activity still remained substantially below normal.
Anything that contains any form of sugar-including sucrose, fructose, and corn syrup -- can impair neutrophil activity: The worst offenders are candy and sweets, which are almost pure sugar and have virtually no nutritional value.
So if you're tired of getting sick so often, do your immune system a favor and cut down on the sugar-laden junk food. You might find yourself fighting off the colds more effectively.