heavy training, and particularly stacked sessions, releases large quantities of the 'fight or flight' hormone, cortisol
one of the nasty effects of this is that it suppresses the immune system
lots and frequent doses of vitamin c helps (it washes through the body quickly, so a tablet every morning doesn't help much) (also, other citrus chemicals such as bioflavonoids aid its absorption, so a tablet every morning helps even less)
eat an orange every few hours or something
leaving more than 6 hours between training sessions allows cortisol levels to susbside, and a quick sleep helps even more (so morning and night, rather than morning and lunchtime)
glutamine imbalance has been linked to suppressed immune system too... there's some evidence to suggest that glutamine supplementation can to some extent counterract cortisol build up when training hard or racing