Flu

Why are you more likely to catch flu when it's cold? I.e winter as opposed to the summer.

It's because your body spends more energy keeping warm against the cold leaving you with less energy later which lets micro organisms attack and your immune system is in a weakened state so it succumbs more easily to disease than if it were warmer.  But according to Wikipedia it's really because people stay indoors when it's winter so you tend to be around more people who have colds and that's why you get them when it's cold.  Fuck Wikipedia.