Hobby Tip: On My Word, Unleash Nurgle!

 It's somewhat sloppy and disgusting work (isn't that the Nurgle way of it?), but here's a few ways to get your plastic or metal figures looking like they've given over to Papa Nurgle in no time. (For those not familiar, Nurgle in Warhammer 40K is the Chaos god of disease, decay, pestilence, entropy, and fearlessness of death.)

Supplies for adding disgusting texture to your figures:

•  You'll need some type of PVA glue. I'm partial to Elmer's Wood Glue as it's a bit "stiffer" in consistency than regular White Glue and comes in larger containers by comparison especially if you buy it at a DIY or building surplus store - then again, you can probably find cheap (as in inexpensive) glue, like Elmer's or another good brand name, at some of the higher end dollar stores (Dollar General, Dollar Tree, etc.) especially if they have a school supply section.
•  You'll probably want some sort of thickener, which ought to serve the added bonus of giving your infected areas some gross texture as well. Anything along the lines of talcum powder, baby powder, or extra-fine grit will do - ideally you want something inert and non-food based (although I've always used cornmeal as a basing material, and I've never had any figure bases turn, um, Nurgly). The thicker the "grain" of your material, the more textured the final result will be - in the example pictures, I used a mixture of fine grit, cornmeal, and baby powder.
•  Last but not least, you'll want to pick up an assortment of smaller-sized beads. If you get coupons in your weekend paper (JoAnn's) or subscribe online (Michael's, Hobby Lobby) you can probably get 40% off a decent-sized container of seed beads - the kind with tiny holes in them so as to resemble pustules on a figure. In the example pictures, I used a mixture of the larger dessicant beads from a few packets of silica gel (the sort you find in new boxed home items or clothing pockets when bought at the store) as well as the tiny plastic ionized beads from the inside of a water purifying pitcher filter (they are a pain to separate out from the activated charcoal, but well worth it!)

Here's the step-by-step on applying a disgusting texture to your figure:

•  Sort out your beads first, making sure you have the sizes and appearances you want - maybe even divvying 'em up in groupings according to where you'll be placing them. (It's not rocket science, granted, but I'm all about having stuff at hand and ready to go when you need it as opposed to sorting through piles of beads while your glue slowly dries unused on the figure.)
•  Mix some powdery thickener into your PVA glue and apply it in a smallish amount where you want your pustules to sprout, preferably not with a good paintbrush unless you want to accidentally make Father Nurgle extra proud.
•  Stick your beads where you want 'em, making sure they're aligned (sprouting?) just the way you want the final product (crop?) to look.
•  Slather on more of your glue and thickener mixture around or even on your groupings of pustules, trying to be sloppy but careful - there's an oxymoron for you, but I'm thinking you can picture what I mean. You probably want to steer clear of the tops of your pustules-to-be, unless you want the areas near the top of the sores to be grainy textured - I'd think smooth would be the way to go (like pus-distended skin stretched sickeningly taut by the diseased innar... er, yeah).

 As the glue dries, it'll tighten everything into place and your powdery/gritty thickener ought to generally make a mess of the appearance around and between your pustules, as well as smoothing out reasonably well where the diseased areas meet the "unblessed by Papa Nurgle" portions. You might make several applications (like I did in the example pictures), using more coarsely grained thickener to glue the beads down and then going back with a thickener like baby powder to smooth things out a bit.

A mixture of PVA glue and baby powder:

A mixture of PVA glue, fine grit, and cornmeal:

Adding a look of disease and decay to your figures:

•  Gather an assortment of fine drill bits, the smaller the better. (In the example pictures, I used drill bits from 1/78" all the way up to 1/8" in size.
•  Make sure you have a sturdy base to mount your drill bits for use - a good pin vise should have a chuck for extra-small bits or be otherwise adjustable to hold even smaller-sized ones. If your pin vise is not nearly as adjustable for tiny drill bits (my first one wasn't, for example), you can take a short cut-off length of dowel and make a relatively deep hole in it a little larger in diameter than a drill bit you want to mount (I used a pair of locking pliers to grip one of my smaller drill bits initially - it was hard work, but I finally got a nice deep hole to set my 1/78" bit into with room to spare for glue.) Once your preferred drill bit will fit in the dowel, try roughing up the non-spiraling end of it with a file or sandpaper to ensure a good grip for glue. Slather the roughed-up end of the bit with glue and set it in the hole you drilled - I'd recommend epoxy glue since CA glue (AKA "Super Glue") is overly brittle, but even something like rubber cement would work in a pinch.
•  Drill holes in various sizes across the areas you want to look especially diseased on your figure. It's probably best not to clump larger holes together, but a larger one surrounded by an assortment of smaller-sized ones generally looks good. Make sure not to repeat any particular pattern over the surface of your figure, since this is ideally supposed to be random.
•  Tidy up any burrs on the edges of your drill holes with a quick, light drag from a scalpel or hobby knife (unless you want the holes to look somewhat inflamed as well).

And here's the finished detail on a figure, a combination of added texture and tiny drilled holes: