Thought this would be obvious: I was not talking about stuff that makes everybody ill. The body does not adapt well to many other things, but still you can get used to many things that make people ill on first contact.
Why do so many travellers get sick first time they enter a country with different hygienic standard, where the local population has no problem?
btw: I lived 2 years in central Africa, just returned from a 'local meal' in Luanda/Angola.
The point is that you don't want to try to be like the locals. The locals in developing countries are not healthy and do suffer from poor sanitation (diarrhea alone is responsible for 8% of all deaths in Asia -- WHO stats). You need to be sensible and avoid certain things.
I just got back from a month in China and didn't get sick despite eating many things that'd make a typical westerner puke. I lived in China for several years previously and had learned the hard way about food safety.
Don't think the locals are not getting sick and dying either. They are. Diarrhea is one of the leading causes of death in the developing world.