What kind of "infection" are you looking for? The common cold? West Nile virus? Some kind of "plague of evil"?
You could use the taint rules, which I think would be more interesting for a "plane of negative energy", here's the rules from UA:
http://www.dandwiki.com/wiki/UA:Taint
What do you want the results of getting infected to be? Do your players get the sniffles? Do they go frothing mad? Do they slowly turn evil? Do they mutate into horrid abominations?
IMO: any time a player is in an area where disease runs rampant, be it a decaying forest, a mucky swamp, or a plague-riddled city, any open wounds are going to pose immediate risk of infection. They may not even get a save, the DM might simply make an additional roll behind the screen to see if they got infected, and then make them make checks later once it's set in.
The simplest way to infect your characters quicker is to simply make an additional die roll every time they take damage from an enemy. That roll should correspond to a list of possible diseases, with some room left for of course, not getting one.