Change the sickness instadeath to a HP-based form of damage, since it is very obscure about how long you have left to live and can be drastically reduced.

• Instead of a death timer, apply a penalty to HP regeneration rate, with this penalty being variable and slowly decreasing as you recover from the sickness. This is nice because it allows for monsters to have a sickness attack in the early game.
• This penalty can be large enough to make your regeneration rate negative.
• Getting hit by a sickness attack while already Ill not only increases the severity/duration of your illness, it deals some extra damage proportional to the severity of the sickness.
• Severe enough sickness only partially times out - you will need to take some actual self-healing action like quaffing healing potions, praying, casting cure sickness, applying a unicorn horn, etc. to be completely cured.
• Unicorn horns no longer cure the entire illness all at once - they will only reduce the severity (which might cure the illness all at once if it’s mild).
• Being sick for a long time may cause vomiting, Con loss, Str loss, and occasionally hallucinations.
• Sickness will eventually go away on its own if it’s not bad. Maybe look to FIQHack zombification disease for specific mechanics for how this could work.