Virus
$1 Action-Attack
Each other player gains a Virus.
As with Rats, there are 20 Viruses in the supply at the start of the game.
I've always wanted there to be an attack that hands out more copies of itself, but I could never get it to work. The thing is, there has to be a reason for the first player to go out of his way to buy one. You can do that with an on-buy effect, but that feels a little clunky.
Anyway, I think this is the most interesting formulation I've come up with of the concept:
Funky Rats
$4 Action-Attack
+1 Card, +1 Action. Trash a card from your hand. Each other player gains a Funky Rats.
Not really something that would ever get released an official expansion, but it's fun to think about. It's a $4 cantrip trasher with the drawback of giving everyone else a $4 cantrip trasher, but with the bonus of that $4 cantrip trasher giving you a $4 cantrip trasher, etc.