In Iraq For Sale, a movie about war profiteering, one of the scenes is an explanation of water purification facilities provided to U.S. troops. The person explains he worked for a contracting company installing water purification equipment for the troops, and he dutifully tested water samples once the system was installed, and found the water was off the charts for bacterial etc infestation. Bad water means sick troops, and sick troops can't fight so well, and the diseases they catch might be long lasting.
Mess hall boss arrested for sickening Iraqi troops describes a recent incident of food poisoning by a military contractor. The food poisoning sickened a bunch of troops. These weren't U.S. troops but Iraqi. No doubt it's the same contractors that serve U.S. troops, however.