My kids don't get sick very often, touch wood. But I happen to think there are things you can do to help with this. The obvious ones are a diet high in antioxidants (the brighter coloured you fruit & veg, the better) and regular intake of live culture yoghurt is another. But the main, most important of all is teaching your child how to wash their hands properly, using liquid soap. Front, back, thumbs, wrists, between fingers. Make sure your school has liquid soap dispensers in every toilet block (believe it or not, some schools scrimp on this basic hygeine standard). Also teach them to cough into the crook of their elbow, and sneeze into a tissue that immediately goes into the bin. This prevents you from reinfecting yourself/family members over & over with the same virus.