if it infects its own creatures its a frekin tank
it infects everything but the players and this includes it's self. So with the upgraded card it'll get 3 turns of use without some hp boosting buffs or healing from an angel. I ment it that way as a way to control an abuse of it after all a plague style attack every turn would kill the strongest deck in short order. Also look at the name if you have something thats infectious it's not gonna be very controled now is it?
Also i should point out y'all are only looking at it as a stand alone. What would you get is you used a Quintessence on it right after it showed up? Well for the unupgraded you would get 3 turns of the effect. And the upgraded version you would get 6 turns of the effect.Why is this you ask.Well it's simple with Quintessence in play the zombie is now immortal and will not get hit by it's own effect.That why it comes into play infected without it you would have an uber powered card no matter what it's hitpoints are. Now here is a cheap rainbow combo.
Infectious Zombie 6
+ Heavy armor 1
+ Upgraded Quintessence 3
and that gives you 12 turns of effect if you have 24
so yes there is a BIG reason for the effect to hit everything but the players and borrowed/immortal.