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I tried to build B/U infect but found it a little too slow for meta game at my FNM. Mono black seems faster and more effective. Infect needs to be aggro to win consistently.  If you have to keep mana open for counters it slows down too much and your dead.  I splash blue with {drowned catacomb} just for {tezzeret's gambit} because sometimes you need a card advantage and it gives a poison.

Sorry. I would swap out your metamorphs for {Phyrexian Crusader} costs one more but 2/2 first strike with potection from white and red. Keeps all the {lingering spirits} from chump blocking you.

I would use {Tezzeret's Gambit} instead of contagion engine. Cheaper you and get two cards. Also I would add {livewire lash}. When you use {artful dodge} it deals two poison or if opponent tries to target they get two poison. I always throw in {mutagenic growth} in my infect decks. If your creature hits costs two life to add two poison and if you have lash equipped that's 4 total plus the 2 pump plus whatever the creature is.  I've hit for 7 poison with a {plague stinger} on turn 4!

I prefer blade splicer over the guard, cheaper and you get a bigger token immediately instead if waiting for the guard to die. And I would absolutely run parallel lives in a white green token deck. When you cast fresh meat after your opponent wrathed your birdies and parallel lives is on the board you will be a believer. Done it many times.  Not sure about your meta game but you should run celestial purge in your sideboard. And definitely pick up a couple elspeth; just hold back a virtues if she is on the board.