Let’s crunch some numbers.
Just to take something as a baseline, the Black Death of Europe (1348-1350) killed about 33% of the population of Europe. That’s approximately 1.1% of the population per month. Now, I know that’s an imperfect figure, because it doesn’t take into account births, or other causes of death, but given my reading on the subject, it’s actually not too far off, when brains mightier than mine apply themselves to the death-rate problem during the Black Death.
My question would be, how many clerics would it take to stem the tide of such a virulent disease? Would a Black Death be possible in a D&D world, or would the grace of the Gods be enough to stem the tide of infection?
Consider this: the clerical spell Cure Disease is a third level spell. That means only a fifth-level cleric can cast it, and it requires a 9 hour rest time (minimum) to recover (plus 10 minutes to actually cast the spell). Assuming those fifth-level clerics are casting their spells at the maximum rate, that gives:
720 hours per month ÷ 9 hours per spell = 80 spells per month
Wow; now that I actually work it out, that’s pretty measly, and that’s working full-out, with nothing else going on. And that’s only counting fifth-level clerics!
But paladins have the power to cure disease as well; once per week at fifth level. So count paladins as 1/7th of a cleric. Fifth level cleric, that is.
To stem the tide of the Black Death, you’d need 138 fifth-level clerics (or their equivalent in paladins, at the going rate of 7 paladins per cleric) to cure the new cases of disease, per one million people:
1,000,000 people x 1.1% = 11,000 dead per month
11,000 ÷ 80 = 138
The question becomes, how many clerics are there in the setting? If you’ve got 138 5th-level clerics per million people, you can hold off an epidemic as virulent as bubonic plague. Anything short of that, and you’ve got a problem. The extent of the problem is directly proportional to how far short of 138 5th-level clerics are available, per million people.
In a city the size of Rauxes (pop. 41,000) or Greyhawk (pop. 58,000) that translates to 7 or 8 fifth-level clerics, respectively. I think that is entirely reasonable. In a more rural setting, where higher-level clerics are more scarce, the story might be different. Hommlet, for example, could be served by Terjon and Jaroo (twice as much as the small village needs) and come out unscathed.
Ultimately, unless the DM invents some sort of magic-resistant or extremely virulent (beyond the bounds of the black Plague of Europe) plague, clerics of a typical campaign setting should be able to handle the problem. Of course, it wouldn’t leave them any time to do anything else, which might be an opening for some enterprising Arch-Daemon to wreak havoc…