Probably from decreased health. Eating specific foods (such as anything in the Junk Food category, or mutagens) will lower your health, and make you more susceptible to cold and flu. The best way to avoid this is with the Leukocyte Breeder System, otherwise, find a way to increase your health. I actually don’t know any way to do this, but I think Dandelion Tea is supposed to help.
EDIT: The “Self Aware” Trait allows you to tell what your Health Stat is whenever you wake up or activate the trait.