Say you're fighting dire rats and get bit 9 times, failing your save each time. If their incubation period rolls are evenly spread, do you take 3d3 Dex/Con damage after 1 day, then possibly 6d3 damage the next day, and 9d3 damage starting once every "instance" of the disease is active?

If that's not it, then how do diseases stack, if at all? It seems somewhat silly, but I can't find anywhere in the DMG that says it doesn't work like this.

  • \$\begingroup\$ I guess your question should be can the SAME disease stack? because in the example it seems like your character would be exposed to only 1 disease (Filth fever). \$\endgroup\$ – Maxime Cuillerier Apr 5 '20 at 4:51

You can only be infected by a particular disease once at any given time

From Disease:

When a character is injured by a contaminated attack touches an item smeared with diseased matter, or consumes disease-tainted food or drink, he must make an immediate Fortitude saving throw. If he succeeds, the disease has no effect—his immune system fought off the infection. If he fails, he takes damage after an incubation period. Once per day afterward, he must make a successful Fortitude saving throw to avoid repeated damage. Two successful saving throws in a row indicate that he has fought off the disease and recovers, taking no more damage.

  • For the first part (getting infected) you probably already figured out that if you succeed on the first one, you gotta succeed on all the next attacks to prevent being infected, so for that part it does ''stack''.
  • The incubation part is when the disease is building up in your system, you either catch the disease or not, you can't have it twice in your body (real life logic and no more info in the rules) so after the ''first'' incubation period (1d3 days for Filth fever) (you take 1d3 Dex, 1d3 Con damage) you now have the disease and must make a saving throw once per day afterward, so ignore the other wounds sources of the infection, you already have the disease and cannot recatch it (nor having it 2 times as bad) while already being infected so no other incubation periods.

If your question is really "Do diseases stack?": if they are different diseases, then yes, they stack - as shown in the spell Remove Disease, which states:

Remove disease cures all diseases that the subject is suffering from.(...)

I could not find better RAW examples, I guess rules were intended as such.

  • 1
    \$\begingroup\$ IIRC There's an entire class based on stacking diseases - the Cancer Mage. \$\endgroup\$ – J. Mini Apr 5 '20 at 13:31
  • \$\begingroup\$ @J.Mini In the Book of Vile Darkness. \$\endgroup\$ – ValhallaGH Apr 5 '20 at 14:05

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