Cat infects human A, who feeds cat in a fit of unprecedented affection. Human B sees another human with a cat and wants to be their friend. Repeat until cat friend puddle is too large for local flora to sustain. Human Z invents agriculture, attracting rats. I'm not sure this is terribly plausible, but I do have a cat on my lap.

That civilization bootstrapped due to brain chemistry changes via human/cat association.

This catspiracy goes all the way to the top!