Another theory that caused the extinction of the dinosaurs is disease. As the Cretaceous Period went on, more and more land bridges started to appear on the earth. Land bridges made it possible for the dinosaurs to cross from one place to another. The change of climate caused the oceans to dry up. The dinosaurs were able to walk across the land that became exposed. As the dinosaurs searched for food they would walk across these new land bridges. But land bridges brought more than dinosaurs looking for food into the new land. They brought all of the diseases that those dinosaurs had inside of them. The diseases did not hurt those dinosaurs because they were immune to them, but to the other dinosaurs they could be fatal. In most cases, disease in a species is not fatal and is a positive thing because it will wipe out the weak, so only the strong survive to pass their genes on to the next generation, but in the dinosaurs' case it might not have been that way.