Quick tidbit: Eating a high sugar snack or meal will limit your immune system tremendously for about 2 hours. High sugar also means simple carbohydrates. It does not mean complex carbohydrates - such as fiber.
Why does this happen:
Good question. I've been resesarching why this happens and I'm finding it difficult to find a clear cut answer. However, I did find research proving that sugar does indeed limit the immune system.
One study concludes: "Acute, short-term hyperglycemia affects all major components of innate immunity and impairs the ability of the host to combat infection"
It's Fall. It's windy. It's getting cold. Limit sugar. Stay healthy.