One of the most common causes of 'tummy bugs' in places like Egypt/Turkey and other really hot destinations is people coming in from outside where it's 40 degrees and drinking ice cold drinks, it puts the stomach in to total shock and within hours you've got severe cramping and the runs. Keep a bottle of cool/tepid water that you can quench thirst with and save the really cold drinks until your body has come back down to a more normal temperature.
The other is people just not staying out of the sun between 12 - 3. Do remember this is Africa in August and a symptom of sunstroke is v&d.
As vix says, no ice cubes, careful with salads, peel fruit. Make sure meat is well cooked.
I've travelled extensively in Egypt, Turkey, Jordan, Tunisia and Morocco and only been ill in Morocco from under-cooked egg which I knew I shouldn't have been eating so totally my own fault. I eat street food, eat in local restaurants and certainly have showers. Never been ill from a shower, even in Luxor when I had cockroaches dropping on my head.
It is perfectly safe to dip your toothbrush in the tap water, just don't drink it or rinse with it. The water is not intrinsically poisonous - but our bodies are used to our home bacteria not the ones the Egyptians are used to.
If there is any chance at all you can get the Hep A shot before you go that also lends you protection.