Nausea and vomiting is controlled by both central and peripheral nervous mechanisms.  The brain has a center devoted to nausea and vomiting and receives signals from the enteric nervous system.  Bad or contaminated food or poorly prepared food will be rejected through a mechanism that elicits reverse peristaltic force, in order to purge the body of the offending food.  Some individuals may have sensitive stomachs, some may be allergic to certain foods, still others will send aberrant signals to the brain.