The party is walking through the forest at night when they come upon a clearing. Half a dozen black-robed corpses lie scattered across the ground, and a pentagram of blood is shoddily drawn in the dirt.
It seems these people summoned something they could not control. Whether or not the demon returns is up to the GM, but it would be just as rich if the demon never returned, and for the rest of the night, every stray sound or odd shadow will be jumped at!
A party of adventurers walk along late on an open plain, on a moonless night. Abruptly, War screams, the clanging of metal and death-cries are heard. It is an open plain, and nothing is seen, but the sound of a huge battle is all about them. The sounds continue for a half hour before stopping as suddenly as they started. What was it? Perhaps the ghouls of a long-gone battle, reliving their unfortunate last memories...
In some remote regions of the lands, voices of spirits and ghosts can be heard whispering in the wind, and wispy movements can be seen out of the corner of the eye.
Pick up a pathology/microbiology book, and add the diseases/syndroms to your game. Your players will freak out.
Coughed at? Diphteria. or tuberculosis?
Drunk from a muddy stream? Or just walked through? Well, let us introduce you to schistosomiasis...
Hit by a stream of energy? Too moch mana is ust like too much radioactivity - perhaps all mages could develop cancer early if too powerful...