I am thinking about heading down to South America, namely Brazil and Argentina the end of January for 3 weeks. This may seem like a dumb question but from what I have read online it is making me think that you have to be very very careful not only in what you eat but how you eat your food there. Basically I would like to know it is as bad as they say. The last thing I want is to be laid up for a number of days due to travelers D. I've traveled through Eastern and Western Europe but it's not the same. I thought it was time to branch out my travels.
I've been inflicted with traveller's diarrhea in both Central and South America. For me, it hasn't been so bad to stop me from doing things. There have been a couple of days where I haven't ventured too far from a washroom, but generally it just means that I have multiple bowel movements in a day and that they are "unformed" (stole that word from the CDC - much better than what I would have normally called it).
Make sure you wash your hands throughly. I have started to carry alcohol based hand sanitizer (the kind that doesn't need water) and wet-naps with me, and ensure that I wash my hands before and after.
The rule with what to eat is usually "boil it, cook it, peel it or forget it." This is pretty sage advice. (That being said, I have has sushi and salad in South America without any ill effects. The key was going to nice, mid-scale or upscale places, not dirt-floored market stalls. However, I did get TD from peel and eat shrimp at a 5-star all-inclusive in Mexico, so there are no guarantees.)
Check out the CDC site for more details: Traveller's Diarrhea
It really doesnt matter how safe you are--you will get sick. However, your body will build up a tolerance for the new bacteria whereby youll pretty much be able to eat and drink anything after a bit of time. I helped train my stomach by trying the tap water in each country--it's not as bad as you would think.
I went to India after a while in South America and was able to eat anything and everything without getting sick. Just need to toughen up your stomach a bit.
Some the best food youll discover will come from the most sketchy establishments. Also, there will be situations when you only have one option for food--eat it, and hope for the best.