The big problem with this card is the HUGE delay involved just to do any damage.
It will effectively be 3 turns before this can do ANY damage to the creature you want to kill, and even more to actually kill it.
Compared to instant effects that you get from other cards, it just isn't worth it.
Example:
Say that the opponent's creature is a... Photon.
Turn 1:
Opponent plays Photon, does 1 damage.
You play Parasite.
Turn 2:
Photon attacks. 2 damage total. (Possibility of Parasite dying or rendered useless).
Parasite uses ability.
Turn 3:
Photon attacks and dies, 3 damage total.
Compared to other methods, this kind of CC is really bad, since it's slow, and is vulnerable to CC as well. It's just not worth it for the duo quantum investment and lack of other uses.
Compare the above example to this:
Turn 1:
Opponent plays a Photon. 1 Damage.
You play Lightning. It dies.
See the difference?
Now, there's one change that would make it awesome. Parasites feed of of their hosts, right?
: Infect target creature. Gain +1/+1.
There. Appropriate buff, improved versatility, thematically appropriate...
What say you?