Training and Illness

Hey guys,
I'm right in the middle of training for my first half (nov 16) and suddenly, my immune system is taking a nosedive. Last week I was getting really bad tension headaches, early this week I battled a crazy head cold, and now I've got some weird stomach bug. Could this be related to my training intensity? I'm trying my best to run through it, but have had to sacrifice some miles. And I'm afraid this stomach bug is harmnig my proper fueling ;)
I'm following the plan perfectly so I know I'm not overdoing it.