Cursed - but by what?

The locals will give you the antidote for bone dust, but you must request it from the posioned sim, and only on the final stage will the option be available, there is 3 different stages, there is a higher chance that your sim will make it out of the poison curse before the third stage, keep in mind, that there is three levels of poision that can be released. Some poisons are released by the end of the first phase, others the second and even thogh varey rare the third stage the death by poision stage is the only stage you need the antidote to get out of. On the first and second stage you can get rid of the poison early by paying respects to the statue thing at the market place. If my sim gets poisoned I just keep them poised because once you have one curse you can get a new one from any of the traps/switches, tresure chest, or dirtpiles.

Edit: Note the bone dust must be in the poisoned sims inventory to get the option.