Some argue that colds being more common in winter is proof that viruses cause them since people supposedly spend more time huddled together in winter and so pass their viruses onto each other more easily, which is complete bogus in modern society, when people actually tend to be far more isolated during the winter than the summer.
-hypothyroid people have lower temperatures, which the cold weather makes worse and the body can't raise temperature to kill pathogens. also, when hypothyroid, one is sensitive to insults from the environment like allergens
-heaters make the air very dry, and our mucous membranes become dry as well, and function less in the ability to trap foreign particles; these foreign particles need not be pathogens, some cause an allergic reaction that helps weaken the immune system. if one's hypothyroid, he is much more susceptible to the allergens
-people living in apartments with a common heating system or a common ventilation system are not living in isolation from each other, as far as sharing the air they breathe; I lived in one before, and every year I was helpless against the flu. perhaps now I would survive the winter, since I'm no longer hypothyroid and I can take the insults from sharing the same air as other people