Have I told you about the "Mouse is the Reason" MHA theory? It's a theory that I found on YouTube one time that I just remembered about.
It's basically a theory on how mice had this disease and spread it to Humans, making them gain Quirks.
This could also explain how Nezu has high intelligence, because he was one of the first mice with the disease and gained a transformation Quirk of some kind.
There isn't much evidence for this theory, but I do still believe it is true. As it would make sense on how more than just one family gained the abilities.
Anyway, have a great day, noon, or night!