Wow! Thanks for the advice, guys.
I think that sand brown suggestion with a very, very diluted pale green wash and a diluted weak tone (or sepia, or whatever) ought to do the trick.
The goal is to make very sick looking people, not dead, for the Infected.
There's more freedom with blood, dirt and stuff for the Mantic undead.
What's with people and our obsession with zombies, anyway...? Seriously. That's disgusting.
Now please excuse me while I go put on another episode of The Walking Dead. I need to know what happens next.