When you eat food that flies have laid eggs on or in which are maggots you can become sick and possibly have stomach pain, vomiting, diarrhea and other symptoms of food poisoning.
Usually eating food that has had maggots laid in it is not a serious health issue and you'll usually just be sick for a few weeks or so and then poop out the maggots.
The stomach acid should kill the maggots if they get into your stomach.
When I was a kid I accidentally ate some food that had maggots laid in it and only noticed after I got a few bites of the food down.
I was so disgusted and my mom took me to the emergency room to be checked out and they couldn't do much for me and said that the stomach acid should kill the maggots.
I had vomiting, stomach cramps and felt really awful for awhile afterwards for a few weeks but then I got over it.
If you do notice your food has maggots in it then throw it out but if you did happen to eat a maggot or two then the best thing to do is wait it out unless your health gets worse.
However in rare cases some people can get bacterial poisoning from eating maggots in food so if you think you've been bacteria poisoned see a doctor or go to the emergency room for treatment.