do they require medical exam before you get your work visa in china?

Thabso, generally speaking, if the officer does not require, you don't need to submit a medical exam record when applying for the work Z type. However, after you enter PRC, you need to show a medical report when handing the temporary residence permit. According to relevant rules, foreigners with HIV + can live in PRC, but in real practice, it's still likely that they will send you back.