Malignant cells on your field aren't always a bad thing. If your opponent doesn't have a shield, or if you have certain cards (say, feral bond or fallen druid) they can be a HUGE advantage. Some decks even use aflatoxin on their own creatures.
For instance, imagine if I were to use this card in my mutantbow deck, which contains both fallen elves/druids and feral bonds. I could effectively insta-kill my opponents scariest creature, use said creature for a few turns, and then get an endless supply of weak creatures to feed my feral bonds and make super mutants. All for just 1 card and 6
Having the creature damage you might work to balance things, although some creatures (nymphs etc) are more useful for their abilities than as damage dealers.
Putting a doom clock on the creature instead of poisoning it might be good to . . . after all I don't think this would ever be used on weak creatures. Maybe even make it so the creature returns to it's original owner after a few turns? You might get less objections to "insta-killing" that way . . .
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http://www.pixlr.com/editor/ is a pretty decent free photo editor, and so is
http://www.gimp.org/downloads/ )