Hook: The players are approached by a travelling apothecary on the road. They were recently attacked by bandits and could use a little bit of healing.
Line: The apothecary is willing to help out someone that has cleared the road of some of the bandits and offers them a healing potion for a very reasonable price. Don't make it too sweet a deal, otherwise the players might become paranoid.
Sinker: Unbeknownst to the apothecary, the potion had been switched with a low-grade poison the previous night by someone (he doesn't know who, but it could only have happened the previous night, as it was the only night he fell asleep on watch).
The poison does not take effect immediately, it even heals the character's wounds a bit (stops any bleeding and cures 1d4 HP/SDC). The bad things start to hit that night. The player who drank to poison will suddenly double over in pain (takes 1d4-1 points of damage) and be unable to perform any actions for 1d4 minutes. Each minute, the poison will damage the character for an additional 1d4-1 points of damage until duration expires (if the result is zero damage, then the character only feels pain, but takes no damage).
Obviously the players will need to act quickly to save their friend (a negate poisons/toxins spell will work fine, as will a regular healing potion). Once the initial shock has passed, it should be obvious (if not, point it out) that it was the apothecary's potion that caused the problem. He will need to be tracked down (an easy enough task, he will continue on the same road in the opposite direction as the players and he can't move very fast as his horse is hauling the cart of goodies).
The apothecary is appalled at such an act of treachery and will apologize profusely (even refund the money if the players demand it, but only if they demand it). He mentions his camp of the previous night and will lead the players there in order for them to be able to search for clues.
GM Notes: This could develop into a full fledged campaign quite easily. Just work to answer these simple questions. First, who would have reason to poison the character? A past villain they defeated? A new villain rearing his ugly head? Or perhaps someone is trying to slowly commit genocide against the "good" races (elves, humans, dwarves, and gnomes). How many other potions have been planted and where? How many people are there and how much of this poison is there? Lastly, how do we defeat them?