Most likely you have not left China for over 10 years. Normally the antibody to resist harsh conditions in China will last for about 6 to 10 years. So you are still ok going back to China after living overseas.
After 10 years, your body has fully adjusted to the clean environment in developed world, then you will be much more likely to be hit with illness when you back to China.
Just hard earned experence to share with you. I'm lucky because my wife is a doctor
