Even the switch from instant-kill to infection counters was a good start towards balancing the card. That way, any creature played will get at least 2 attacks, the one the turn it comes into play and the one the infection kills it.
I haven't thought this too much, but it seems like half HP for 5 infection counter ability wouldn't be too OP.
One thing to remember during card creation is that it's always better to err towards cards being underpowered. A card is weak, it doesn't get used, not too big a deal. A card is too strong, it changes the entire game, almost forcing people to use it (see 2-turn sundial).