[RPG] When does a PC become aware they’ve contracted a disease

1. How do I discourage players using the knowledge that they have a low roll to influence character decisions?

Be up front and honest with them about not Meta gaming. It is meta-gaming using the knowledge of a low roll to influence your in-character actions/reactions/thoughts/etc. I usually say just what you said, something along the lines of, "You think she seems to be telling the truth." If my player then goes on to think she is lying, I do one of two things:

• Either gently remind them, "Your character thinks she is telling the truth and would not remain suspicious."
• Or, allow them to roll insight again, but on a different bit of information hoping for a higher roll so it's definitive that the NPC is truthful (or not).

The first option is always better, and usually in the long run makes for more engaging role playing when players can accept the cards their characters have been given. One major tip, when they roll low and you tell them they believe the NPC, try not to act devious or suspicious, your tone and demeanor can influence player thoughts tremendously.

2. Should I, and if so how, tell my players if somebody is actually telling the truth even if they have a very low roll to avoid them thinking they are being lied to and their characters are too stupid to figure it out?

This is a bit tougher... first I refer you back to the first part of my answer to the first question. Players acting on the knowledge of low or high rolls in a way that is discordant with the information you give them is Meta gaming. So, if the 2 options above don't work, then you have to ask yourself one question.

"How will having this player remain suspicious of the information affect the narrative?"

This situation is good for on the fly story development!

• The player remains suspicious, doesn't heed the warnings of the NPC and their actions either result in an extremely challenging fight, or worse may even lead to the death of the NPC. This could cause the player to be more careful in the future or become deeply regretful of his stubbornness.
• The player remains suspicious but the NPC pleads with them, "I can tell you don't believe me, and if I were you I might not believe someone like me either... but please good sir knight, proceed with caution!" This could also lead to the outcomes in the previous example if the Player still ignores the warning. Or they may uncover, incrementally, that the NPC was truthful, after which they might go back, apologize and make a new friend.

If the story really is better served with the character believing 100% that the NPC is not lying and there is no in game way to work around the suspicious nature of the player... as a last resort (and I personally would not do this), just tell them out of character that the NPC is being truthful. I can't think of a reason that you would need to have the character believing that the NPC was honest given the ad-libbed scenarios above, but if you find it imperative, it's your call to make.

...and nothing says the player won't suspect you of lying just because you are the DM. ;) Some players will always be suspicious no matter what as they will always believe the DM is "out to get them".

(And to quote another answer here after reading the stuff posted as I was typing this novel length answer... do be up front with your players that anything you say as a DM to the player will always be the truth. Lies might come from NPCs, but never the DM. And always make sure you are consistent in upholding these promises to your players.)

Here are the rules:

Starvation and Thirst

Characters might find themselves without food or water and with no means to obtain them. In normal climates, Medium characters need at least a gallon of fluids and about a pound of decent food per day to avoid starvation. (Small characters need half as much.) In very hot climates, characters need two or three times as much water to avoid dehydration.

A character can go without water for 1 day plus a number of hours equal to his Constitution score. After this time, the character must make a Constitution check each hour (DC 10, +1 for each previous check) or take 1d6 points of nonlethal damage. Characters that take an amount of nonlethal damage equal to their total hit points begin to take lethal damage instead.

A character can go without food for 3 days, in growing discomfort. After this time, the character must make a Constitution check each day (DC 10, +1 for each previous check) or take 1d6 points of nonlethal damage. Characters that take an amount of nonlethal damage equal to their total hit points begin to take lethal damage instead.

Characters who have taken nonlethal damage from lack of food or water are fatigued. Nonlethal damage from thirst or starvation cannot be recovered until the character gets food or water, as needed—not even magic that restores hit points heals this damage.

Of course, it doesn't cover malnutrition. You would have to make house rules.

House Rules I would probably do

Use the non-lethal damage, but don't take it as damage. Since he is having some sort of food rather than no food, I would allow the non-lethal damage to accrue. After acquiring the amount of non-lethal damage that would normally result in lethal damage, I would instead give 1 point of Strength and Constitution damage. That will show that he isn't necessarily dying, but withering away. Normal food and rest intake would recover the damage as normal.

If you must use an illness

First off, malnutrition isn't really a disease, so I don't think cure disease would or should have any effect. If someone could cast Cure Disease on the target, there is no reason they couldn't cast Create Food & Water.

I would go with a dysentary, cholera or scurvy type of illness. Malnutrition would more than likely result in getting ill from other sources. There is a spell called Advanced Scurvy. You could model your natural occuring scurvy off of that disease.