>>279644592
I believe more that it came from another world like the circus guy said.
the circus guy seemed to know more about the gas and he was literally working with the gas.
He and his circus were hunting the uncle's machines along with the infected in the machines and infected that weren't in the machines even throwing infected into the cannon.

I suspect that Tadashi's grandfather ended up bringing the gas from another world with his experiments and that it was released and merged with nature when the ship sank.