Hey there pal love most of ur work on the undead and the concept of this I like how you add details on the blood and the dull colours seems to fit well even its for a concept. It somehow remind me of road of the dead or lab of the dead one of evil dog's work. Thats preety cool I also notice that you named some of the zombie to the way they behave similar to left 4 dead, but I like to suggest when it comes to concept.

if you would name a zombie like stalker for example make it look more like its stalking so like show a quick tiny draft in the bottom on how it prey its victim, how it hides in the dark waiting, and how it uses its undead ability. Like the Hunter in left 4 dead, its name fits the way it acts it crawls, waiting for the right time to leap and kills its prey when they least expect it like a real hunter hunting animals.

Also try all kinds of other infected people like a police man, doctor, civilian (females, young ones and fat ones), childs, hooker, biker gangs, farmer and many more.

You did a good job dude and thats good to see but this is my suggestion for you to get even more better and to grow as an artist in future so the more you draw the more you've experienced.
Keep up the work buddy :)

-Alex Trinidad