The gods came to an agreement thinking that one day the planet will overflow with water having every continent sink, and all the creatures on the planet will drown including the humans, so they decided to turn all the humans on the planet into merfolk. It was like the events that occurred after the creation years of the Earth.
The virus however would have to be spread based on a specific sea creature located in the Atlantic Ocean, in between North America, and Africa/Europe. Which ever human being dives into that ocean, and gets touched by either virus sea creature that glows bright blue, and has had seashells attached to them, they shall become a full part of the ocean. The creatures the gods placed the virus on are a hammerhead shark, a piranha, a rainbow parrot-fish, and lastly a fish known as the mantis shrimp.
All that was left for the gods to do, was find a human subject, or more to go near one of those fish, and then once they are touched, they become merfolk with the infection.