If you wanna look at it in tin-foil-hat-vision, the Chinese probably were developing some sort of bio-weapon and that went horribly wrong. What's more plausible is that they were experimenting on it to develop some sort of cure or to just take a look at how viruses evolve, mutate or behave and some dunderhead at Fluhan got too comfortable with it.
Either way, this will make Xi and the CCP look either reckless and/or stupid and evil so you can understand why they were supressing information. Just my two-cents tho.