Bacteriophage

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
"this virus parasitizes a bacterium by infecting it and reproducing inside it"
Bacteriophage can't attack unless your opponent controls 2 or more creatures.
At the beginning of your end step and
upkeep Bacteriophage gets +1/-1 counter.