Mercenaries are like the ones in Mass Effect 2. It's a mix between all the major merc groups, including Blue Suns, Eclipse and Blood Pack. They have a mix of tech, biotics and krogan. They would make for an interesting opponent.
Plagued is like husks, melee only. But, when one of them gets to you, you lose health over time. They dont have biotics, tech or any weapons! But, they do come in swarms which will destroy you if you are not careful. However, during the higher waves, Plagued Krogan and Batarians will come in, they attack with shotguns that have Polonium/Radioactive Rounds (From the first game) which are like poison bullets. The 'lore' for this is that during the reaper invasion, aliens all around the Terminus Systems have gone crazy. They're isolation has led to them being mindless, and they will attack anything. You are part of a crowd control squad, sent in to keep them calm, or keep them from infecting the whole galaxy.
There is also a new ammo power, SEDATION ROUNDS, which does more damage only to plague enemies.