I'm a university student, and i generally average around 5-6 hours of sleep during exams. i find that i often fall sick during these periods of low sleep. what can i do to boost my immune system and prevent the flu apart from simply sleeping more?

Enough sleep: You have diagnosed your own problem. Sleep deprivation and stress both reduce your immune system. Getting a flu shot annually would be helpful also. Be sure to wash your hands often especially before eating and drinking and after touching other people's hands. By the way, I think you can learn better when you are having enough rest time.

Nothing: Sorry, but there is no good solution other than better rest. No vitamins or immune sytem "boosters" are proved to be effective. Follow a generally healthy lifestyle in addition to sleep: balanced diet, exercise, stress reduction, high quality personal relationships. But sleep itself is crucial and these things won't make up for a sleep deficit.