Can a person get sick ? if he (she) eat food where previously , dozen of flys standed on, or can get a any stomach illness ?

Yes but . . .: Flies can spread disease (infection) but just the fact that food was left out in the open suggests that it may have warmed up enough to allow spoilage. Check out http://scienceblogs.Com/effectmeasure/2010/01/10/potato-salad-food-poisoning-an/ for an explanation. Also that scenario is often associated w/poor hygiene of those involved in food preparation. Just sayin'!