This would legitimately be the greatest prank anyone could ever possibly pull. It wouldn't even have to be 8 billion, just enough for everyone they could possibly interact with for a few days after landing. And they'd all have to really commit to the bit. I think this is the one thing that can truly unify us as a species.
It'd be even easier to do because astronauts usually have health checkups and quarantine after returning home. Most people would just need to wear hazmat suits.