Probably multiple people that willfully didn't wash their hands rather than one accident, read about how a ton of people at hospitals don't wash their hands for shit, I'm sure some bio science is exactly the same, they're both the same stakes in many ways.
That said definitely not how it all happened ,spread globally, unless the initial cases remained dormant a long time to spread all over across various planes. But... not totally impossible. Still seems unlikely though that there wasn't some intent.