Shaun,
Probably because you have a stronger Constitution than many, something like myself.
Look, .... some of these water bourne bugs are bloody virulent, and you could eat as much yogurt as you like, and it wouldn't suppress it or cure it. If anything, it would probably make it worse..... Over the several years i was living and working in China, i can remember at least 3 occasions where i did cop nasty doses of gut bugs. I can tell you, they ain't nice........
I'm rarely affected by gut bugs normally, but when i have been, it's been badly. I defy anyone, to go to any big city in India (for arguments sake) for more than 15/20 days without getting a good dose of Deli belly or the Bombay trots (or both). And that's taking all the precautions you can. no fresh fruit, no fresh veg, only drinking bottled water, no fresh dairy product , refusing ice in drinks, drinking only bottled beer, etc,etc
Seeing how these trips to China are going to be really important, to most that are going from this site. It's probably going to be far better for them to Air on the safe side and not take unnecessary risks.
According to Lucy, even though the local population have some resistance to these bugs, it's still one of the biggest annual loss of working man hours, to the country.... So, i guess it's up to those going to China to heed advise or ignore it.
David.....