The virus spread through out the world effecting Humans and animals.
The animals evolved from it becoming anthropomorphic animals they could drive cars wear cloths and do anything humans could do.
And at the same time humans who were over 18 when it was unleashed began to pass away from the virus while humans 18 and younger became over grown babies.
There muscles became as weak as a babies they had to crawl and needed diapers now and their skin became baby soft.
On the plus side their aging had slowed down but that also meant they'd be big babies for 10 or 20 years before they could start growing up again and be potties trianed.
Due to this animals have taken control of the world and humans are put into adoption centers for animals to adopt humans to raise as their own.
Most humans didn't like this but couldn't complain to the animals about it as they now had their own language and the humans only sounded like they were babbling to the animals.