Not all mice, some mice, somewhere.

This virus in the form we encountered, spreads among humans best, but it also can spread between a ton of other animals. Most probable explanation is that it originated from civets (second best) and infected a single person with immunity sufficient to suppress it but insufficient to eradicate it. In this person it multiplied and mutated learning how to infect human cells best and finally one mutation allowed it to skip to another person. And then it spread like wildfire because it was already adapted to humans because it mutated in one human for months or years already and just learned how to overcome immune suppression enough to break through.