Natural occurrence is how almost every known virus exists.
Given that prior, "can you disprove the lab" is the Russell's Teapot hypothesis. No, I can't. I also can't disprove it rode in on a meteorite or was beamed in by aliens. And I'm not about to start.
>>btmore+52
And the novel technology to create novel viruses was being actively developed in this lab. They've proudly and openly published results to this effect. Further, they were trying to get tens of millions in grant funding to develop the technology further.