I’ve created a quick document for my current campaign which details a number of generic diseases; I love the basic D&D approach which usually boils down to “will be fatal in X days”, but I also like a bit more flavor both to the mechanics and the descriptions of the disease’s progress.
The basic concepts are that the GM determines which type of disease to use and its severity, although this should be fairly evident depending on the source (Giant Rats feels like the Raging Disease (which is always Deadly), whereas the Heucuva of the Barrowmaze my PCs are about to encounter cause Severe Infectious Rot).
The optional “infectious” rules would mean those who travel with an infected person might be forced to Save at the same time they do, or contract the disease. This can quickly lead to disaster for a team of adventurers, of course, and might also mean a bit more of a hassle than it’s worth. I don’t think I’d apply it except in special cases.