Head cold is probably a symptom of your body being a bit overloaded due to your requiring more of it - i.e. when you start hitting the metric century+ mark (at least in my personal case) you can tax the immune system if your out beyond the 2-3 hour mark. Combine this with piss poor nutrition and your looking at a train wreck of the immune system, ending up with a bad/severe cold, or something else - flu, etc.