C.T.
There is such a broad number of reasons and remedies with a situation like this...
But food for thought - - their diet and nutrition plays a huge part in their ability to stay healthy. I have 7 children (15yrs and under) and I guard their diet better than my own. I try to feed them only whole foods - nothing processed or preserved. It if doesn't come in it's original form, I try not to buy it. Also, vitamins are very important because nowadays, no one eats perfectly to get all that they need. I give my children vitamins everyday (and not the flinstone type). They get a multi, codliver oil, and very importantly vitamin D. They get others based on circumstances. Sometimes extra C if I fear that a cold may be about the house, and extra zinc. I can honestly say that my children are rarely sick, and if they do come down with something once a year, it is mild. I hear my friends talking about all the illness their children go through, and I feel bad that their children always seem to contract everything out there. 5 of my children are in school, so that's 5x the chance of them catching something at school or bringing it home to the rest of us, but it doesn't happen. I think about them climbing the stairwells and sliding their hands up and down the banisters, and all the germs on the desks as they change from classroom to classroom, and I don't even want to think of the bathrooms... but I have them wash their hands as soon as they come home, and as a rule, no one gets sick.
Just my 2 cents - food choices and vitamins are a very important consideration.
Good luck and I wish you all good health :)