Now, why would you want to include these things in your campaign?
- Local color: some places are just overcrowded, hot, humid sewers, and the risk of disease can really help make a place "real" in your player's minds.
- Ticking clock: if the disease is wide-spread, the NPC clerics might not be able to help everyone unless the PCs undertake a quest. And the longer the PCs take on the quest, the more people will die!
- Added trouble: it's a truism among authors that if you really want to write a great story, after you get your main character in hot water, you drop a rabid badger on his head and watch the fun ensue. The same can be true of your games, so long as your players recognize you're not just dumping on them. (A fine line to walk, in all honesty, but it can really improve your game if your players trust that they will be given a chance at victory and payback.)
- More spotlight time for the cleric, without having to resort to undead. Again.