There is a very good and clear blogpost on Daily Kos about radioactivity in food, and I recommend reading it if you want to know more. It also reprinted this excellent description of the difference between being 'exposed' to 'contaminated', from a Vermont brochure:
When we came back to Sweden all cars wheels were checked with Geiger counters at the border, and we found out that the cloud had changed direction and hit the Baltic countries and Scandinavia. For many years thereafter you could (=should) not each wild mushrooms, berries, and reindeer meat from northern Sweden, due to Cesium 137 contamination. The reindeers eat the lichens which has accumulated the radioactive particles.