While running my last session, I disseminated some information to a player prematurely that could have been fun to eek out over time. The wizard in question had just been bitten by a Werewolf and, failing his Constitution Save, contracted Lycanthropy.
I immediately told him, “You feel something wrong with the wound.”
Of course he gathered what had happened immediately. But how long does it really take for a disease/curse to manifest? Fifth Edition gives Incubation Periods for all three of it’s sample diseases (usually hours or days), a period during which the Player has no symptoms. Unless the player performs a difficult Medicine check, it should be relatively difficult for the character to identify that they’ve contracted a disease at all – let alone what the disease is.
The Incubation Period of Infectious Diseases
I thought I’d take a look at some real world incubation times for comparison. Here’s a link that talks about the incubation period of viral infections, which I think is a good comparison for the infectious nature of Lycanthropy, Vampirism, and DnD Diseases. You can see that it is rare for a infectious disease to incubate in a single day. Those illnesses which do are usually minor, such as the Flu and the Common Cold, and have manageable symptoms. This leads me to my new rule-of-thumb:
Mechanically weak diseases should set in within 1d4 days of contraction. Mechanically strong diseases (and Lycanthropy/Vampirism) should set in in 1d4 weeks.
Should Bestow Curse or Hex have Incubation Periods?
Short answer? No. Since the attacker should gain the mechanical advantages of their spell immediately, there is not a way that magically-induced curses can have incubation periods. I make Lycanthropy and Vampirism (still technically curses, I believe) exceptions, because the attacker gets no immediate advantage from their victims contraction of the illness. We can afford to add an incubation period for RP.
So we need to find another way for a immediate curse to reflect actual illness. I suggest that you tell the PC that they have been cursed (“Shooting pains cut through your chest and your skin grows sallow”), but do not tell them the effects of the curse. Do not tell them if Bestow Curse was cast upon them and the caster applied disadvantage to all of the PC’s dexterity checks. Instead, let them roll for checks as normal and secretly roll a second die. Take the lower result. Players who pass an Arcana check of the Spellcaster’s DC should gain the mechanical knowledge of their curse.
We don’t want diseases to become intrusive and create work for the DM. After all, secret tracking of disease incubation periods is probably not every Dungeon Master’s cup of tea. If you do decide to go this route, look forward to PCs contracting the flu in the swamp and not seeing symptoms until the big fight three days later. Or a PC who contracts Lycanthropy unknowingly and wonders why, two weeks later, he keeps waking up far from his bed.