My most serious health problem while in China was halfway through the trip I had sharp pains in my right knee (there is a lot of walking and a lot of stairs), which reoccurred intermittently through the rest of the trip.
I never got diarrhea, but Mei and I both caught a minor cold/flu in Xi'an (the main effect was to rob us of our energy). Other than that, all was well health-wise.
Of course, I got food poisoning (vomiting, diarrhea) in Canada about 1 month after I returned from China.