Bear in mind this is "PnP Undead". I prefer to not needlessly deviate from the source material and there is nothing there to suggest the disease is spread by anything other than being touched by a Mummy (or bitten by a Ghoul Lord, or whatever).What would make disease especially dangerous (compared to poison) is that it can spread from one character to another. Now that opcode 272 is fixed, use that with a nested opcode 146 (cast a shell spell).
This is easily doable by adding sectypes and using timing mode 9.
[1] Some PnP diseases can only be healed by some more exotic spell, like Regeneration or Heal. But due to the technicalities involved, and to avoid confusion, I'm reluctant to implement this particular aspect of those diseases.
I know, but I'd rather not end up adding two dozen SecTypes. Especially not since the only way you have of obtaining the information of what spell cures which disease is by reading whatever HTML document I prepare (short of trial and error). In PnP you can query the game world and have your DM provide you with the information in some manner, which changes things quite a lot.