Do you have the book of Vile Darkness?
If you want to go for a prestige class, there are several that may work for you. However, I do not think that your villian will meet the requirements of the more intresting ones. Even the most likely ones will likely require feats from the BoVD, so some retroactive modifications may be necessary.
Vermin Lord:
You will get some nice special abilities that are not over powering, and every odd numbered level advances a pre-existing spell casting class. However, you need to be able to cast a particular spell to qualify, and one level of Druid is not going to do it.
Cancer Mage:
The villian is pretty much a walking disease, but the requisites are for a higher level then what you propose (+5 base fort save is what a 6th level fighter has). But the up shot is that it is
Thrall of Demogorgon:
Not much of a disease theme, but some nasty powers, and you can easily qualify for it without dramatically over shooting your players capabilities.
Thrall of Jubilex:
Very gross powers, and with some modifications / name changing, you can have a rather villian that should fit your disease theme. The requirements will put you a bit beyond your ideal level target though.
Anyway, think very carefully before choosing any prestige class. Most of the ones I have seen require you to be a fairly high level, and unless your willing to settle for only one level of the class, then you may end up over shooting your players power level.