The gods were fed up with the way humanity treated nature. The thought came to them that perhaps the best solution may in fact be to delve in to the mythology of humans to find something that would work. Studying the works of fantasy, they believed they found a solution with the Elves. The fair folk had a love of nature and all that was beautiful, and perhaps they would fix what humans had done to the world as a result.
The infection would start simple. Humans would begin to develop points to their ears while their features would become sharper and more elegant later. Finally they would develop magical abilities and a deep connection to nature that allows them to remove unsightly constructs. The infection would spread through two factors, either juices or through magic itself. Magic would be able to ensure that the infection would spread. In the long term, humanity's lifespan would increase, but only when it would not result in overpopulation.
The virus would mutate as time passed and form new varieties of Elves. The Dark Elves, Sea Elves, and Wood Elves among others. Each of them would develop their own societies. This may have problems for the gods' plans in the long run, but they were fine with this. It would still be better than the destruction of the humans.