This condition is called motion sickness and it is not a disease, but an abnormal physiological response of the body to movement. You should take the following precautions:
1. Avoid travelling after a full meal.
2. Take a medication on doctors advice, like Avomine, Domstal, etc.
3. Don't look outside the window at the moving objects.
4. Keep yourself busy by reading or looking at one spot in the bus itself, not outside.
5. Try to chew on a gum or some candy to avoid attention.
6. Sit in the middle of the vehicle, and also not near the window, to avoid seeing outside at moving objects.