Don't drink the water. Make plans on what water you'll drink when there. Be careful about ordering bottled water in restaurants as some places simply re-fill empty bottles from the tap. Don't brush your teeth with faucet water. When you shower, don't let any water get into your mouth. When eating out, don't order something that has been rinsed in water, like a lettuce salad or fruit salad.
This is not bs. I have been down there many many times and I have gotten pretty damn sick on more than one occasion, and I mean sick. Take bottles of liquid Immodium AD - that stuff works great! Take one bottle for every 2 days you'll be there.
I went so far as to buy a bottle of iodine tablets and a silver impregnated portable charcoal water filter. I would fill a gallon container with water, drop in the iodine and let it sit for about 20 minutes. Then I'd pour it thru the filter to remove the iodine. Then I would drink it. If I could, I would boil the water, too.
I know that in some places down there, the sewer pipes (9-inch ceramic) are physically in the same holes as the water pipes. Each have numerous holes in them. When it rains, the sewer lines overflow and empty into the water lines thus contaminating the water. I don't know for sure if this is a problem in Oaxaco, but why risk it?
This isn't jingoism, I know it's true for a fact so be careful. You won't be out photographing if you're spending all your time in the bathroom.
Buy bottled water where you can see the seal is unbroken and keep some with you at all times - like at local food markets. Brush your teeth with it, too.
I've had a lot of mixed results eating out in Mexico, too. Some places are great and others don't practice good sanitation and you'll get sick. To be on the safe side, get all meat cooked well done. Really well done. Get the internal temp up over 180 and it'll kill the bacteria, so I understand.
I've been sick down there and believe me, you won't be walking or photographing if you catch something. The Immodium can be a real life saver, so don't leave home without it. You may be able to get it there at a pharmacy. Pharmacies in Mexico are fantastic - much better than here. You don't need an Rx for most stuff. Also, if you need emergency dental work, the dentists there are all well trained.
Surprisingly, the best food I had was a small hole-in-the-wall taco vendor. He cooked (burned) all the meat right in front of me and served it on tortillas. Delicious and sanitary. I get sick when food suddenly appears from a back kitchen where I don't know what it went thru before it got to me. I also know that restaurants get their meat from uninspected sources so you can't take it for granted like you can here. I had a cheeseburger at a place owned\run by an American couple and spent 2 days in bed afterwards.
All that aside, it's a great place to photograph. I love going there and these concerns won't stop me from going again. Just be careful, ok?
Can anyone else offer practical advice on staying healthy on a short visit?