I've flown several trips to Africa for Delta, including the 8 and 9 day trip, and never got sick, although some on the crew did.

Simple precautions prevent 90% of the problems: don't drink water that isn't bottled, don't eat vegetables or fruits that aren't peeled, ie. bananas and oranges are ok, lettuce is not. (This includes food on the airplane out of Africa) Don't get frozen drinks (because they are made with ice made from poor quality water). Eat at high quality places rather than the cheaper places.