Immune system like any other system needs adecuate resources to keep functioning at its best.
Well It's simple in this case just keep your boddy in activity at least 1 hour of excercise daily.
Have an apple a day, sleep at least 8 hours if possible, drink lots of water, pure water, lots of milk and lots of fruit juice.
Avoid sugar if possible eat meat al least once a week, eggs once a week, keep your system running with yogurt.
Eat everyday vegetables a tomato a day, with some carrots, and if possible some brocoli too, try not to boile the vegetables but water steem them.
Now a bit of your body temperature, try to keep it warm not to hot not to cold if you feel temperature rising just take of your sweater, do not change your body climate to quickly try to do it slowly, if your sweating and cold your body suddenly you are going get the flu.
Wash your hands with lots of soap and lots of water.
And finally once again drink lots of water, juice, and milk, have an apple a day.