Have you been down to the shelter where you might want to volunteer? Perhaps you could take a tour to see what the facility is like and the general health of the animals.

At my shelter, we had 1 room for the adoptable cats, 1 room for the feral cats, 1 area full of kennels for the dogs and then in the back was our hospital where we had 1 room for surgeries, 1 for sick animals, and an area for isolated (usually vicious) animals that animal control brought in.

I would alternate what I did at the shelter everytime I went. I spent most of my time with the adoptable cats in the cat room. All of these cats had their shots and were ready to be adopted...so they were all healthy. There was also a hand sanitizer on the wall, so I always used it between cat cuddling anyway, just to be on the safe side.
When I was working with the sickies in the back, the process was pretty much the same, but I just took extra precaution between handling of the animals and before I left the shelter I sprayed myself down with the nolva-cleanse/H2O dilution spray bottle. When I got home, I would put my clothes in the hamper, leave the shoes outside (or like you said, in your hall closet) and shower before even touching my cats.

Perhaps, if you are thinking of going just to socialize and cuddle the adoptables, I really don't think you should have much to worry about, especially if the adoptables at your shelter are fully vaccinated. It's definitely worthwhile, and the babies at the shelter will love you for it. I always had a soft spot for the older cats and got them out first and brushed them and played with them followed by the younger cats and kittens. The toughest part, I think, was not taking them all home!! Good luck and let us know what you decide.
**hugs**
Karen