This isn't 3.x where there are high level characters everywhere. PCs are special.
A decade ago, back when they did the "Curse of the Crimson Throne" adventure Path, Paizo wanted to do an adventure featuring a plague. And they did an analysis of the demographics, assuming 'normal' numbers of high-level PCs (normal in 3e terms, that is) and the size of a major city.
The upshot was that even in 3e, a plague scenario is entirely feasible. All you need is for more people to become infected in a day than can be cured.
Some things to bear in mind:
- The spells that cure disease generally say nothing about giving any ongoing immunity. A character could well be cured and then infected again later that same day.
- Most diseases have an incubation period where the victim may well be contagious but not symptomatic. That being the case, by the time people start seeking healing, the plague could be more widespread than the healers can deal with.
- Access of magical healing is unlikely to be evenly distributed, and indeed those most likely to suffer a plague are amongst those least likely to have access to healing. The powers-that-be are likely to keep a Cleric on standby at all times, just in case, while also being able to isolate themselves from infection - you only need a fairly small elite to each do that before there's no healing left for the rest.
- For best effect, consider having the plague start amongst a group that are considered undesirable or immoral for some reason. The local society are then likely to respond by indicating that this is just them getting what they "deserve". Then, when the plague expands beyond that group, it's just a sign that these new groups are also immoral... and then suddenly it turns out it affects everyone, by which it's too late for the bigots to realise their mistake...