Infect proliferate (Casual)

First of all, infect win with 10 mark on the oponent (20 on the 2HD), put this in mind.

Don't be afraid of loosing 3 life to cast Invigorate in early/midgame +4 counter on the oponent is worth.
If T1 you can put a forest and the glistener elf, T2, i you attack and the oiponnent can't block, you can cast 2 invigorate for 3 life each and cast a giant growth for a forest, if the oponnent doesn't block the spells basically you win the game.

To time to time in the mid/lategame sacrifice a creature to block another is most valuable if you can proliferate later on.
Each time a damage is done with an infect creature to another, the other creature take -1/-1 for each damage.

This deck is early aggro based. So the early you put counter on the oponent the most stresfull it is for the oponent.
In the earlygame you can ue only creatures to put counter.
But in mid/lategame you can use the proliferate with the Contagion Clasp.

You can use Hand of the praetors and put another creature with infect enter the battlefield to give a counter on the oponent. This is free why not using this.
Seek an available battle for your creature to not be blocked this turn and attack. If the creature remain unblock cast spell that enchance that creature like : Giant growth, Invigorate, Mirran mettle, Vines of vastwood.

The mask or avacyn is to protect a creature with infect like hands of praetors that can put counter directly with only summoning creatures.

Liliana vess is put into this deck for fun, it can discard's oponent's hands, search a card if you have a really bad time or more importantly in late game if you don't hane any creature left and ave sufficient point you can retreive all of them at once plus the oponent creatures.

If your oponnent use a black deck replace et Doom Blade with the Go for the throat
If your oponent use aoe spells or for counter deck blast, you can use the Trigon or infestation to put token with infect each turn that can put -1/-1 on other creatures then proliferate.
Against automill / resurect / life link / regen you can use the Decimator web that can loose life for the oponent, take a poison counter and mills. (When you can't choose, why don't do all of them)