So I'll conveniently copy and paste over the same description of it since I'm pretty sure the rules have not changed :)
Basically Humans vs Zombies or HvZ is a week long campus wide game of tag (invented at Goucher College in Maryland, I believe) that occurs twice a year. The point of the game is to make it the entire week without being "tagged" and the difficult thing about it is that it's a 24 hour event which means that yes, even walking to class you're still playing. This causes mass chaos around doors to buildings because indoors is out of bounds so the minute you step outside you can be tagged. It also means that people will walk incredibly far out of the way just to avoid the quarter mile. I've never participated but it's pretty funny to watch. You can check out the "official rules" by going to http://kdratrit.com/hvz/rules.php
Apparently what I failed to mention last year was that people carry around huge nerf guns or balled up socks and proceed to hide behind buildings, doors, etc and wait for the opposite team to walk by so they can "tag" them. It's pretty funny giving tours while this is going on because when there is a zombie "attack" everyone stops to watch. It's also funny when you walk by someone trying to hide and start laughing because they're get mad at you and tell you to pretend they aren't there :)
And when I say campus-wide, I don't mean that everyone participates. I, for one, have not been defending myself with a nerf gun or rolled up socks on the way to class, it's much more relaxing that way. While I think the whole thing is a tad bit strange, I can understand how it's a fun (and stressful-- constantly being on the look out) way to spend your week 7.
If you're interested in it more, here is a more professional looking video that someone put together to explain it.