Water Under the Foundation

Help! We have HUGE pools of stagnated water under the (raised) foundation of a 5 year room addition. The smell in the house has been horrible for almost a year now and we see little black flies all over. My husband crawled under a short ways this morning and discovered thousands of MAGGOTS in the water which I'm assuming is what is causing the flies. I know alot of the water is a result of poor drainage when it rains but I also believe we probably have a sewer leak somewhere. It smells particularly nasty around the d/w and kitchen sink area. We have a number of problems developing as a result of shoddy remodeling done by a crooked contractor back in 2000. This is the most concerning to me right now. Can anyone tell me what the health hazards are and WHO should I call? My husband is not a handyman and I don't know where to start. PLEASE HELP!