Is It Due to the Heat or a Lowered Immune System?

"I started feeling kind of a little off that night, and by the time i went to bed s Saturday night, i was s wow. You know, i really, really a tanked here," he says. "Once i was able to start getting some water and some food in me, that kind of turned around pretty quick." He's had similar experiences on two separate occasions during his ultra-marathon career. His questions: How much of it is due to the heat? And how tethered are you to your immune system travelling on the road?