This is a great article posted on another site (who reposted it from another site, who took it from another), but I thought many would enjoy it here. It is The Lazy Man’s Guide for Constructing a Call of the Cthulhu Adventure, written by Sandy Petersen, original author of the Call of Cthulhu.
In nature, for instance, a rhino has flies and ticks etc, that live on the hide and live off the blood. Rhino will visit and lie in ponds where turtles will clean the underwater portion of the ticks and a species of bird will clean the the top of ticks and other parasites. What if where the characters camp there were a species of animals that lived off wounds of the creatures. PCs camp. Anybody with wounds on there body are soon covered with small rodent - insect - shadows - whatever. Would immediately see it as a threat I would imagine. But perhaps one of the wounded didn't wake up when it happened, and when they did finally wake they were completely healed. Perhaps somebody was warned of something attacking them but they were able to notice that the wound was actually getting smaller instead of larger. Or maybe they successfully beat the creatures off them and don't notice an improvement, marking this place never to rest at ever again....later learn that villagers send their wounded there but they keep it a guarded secret because they don't want all the attention or traffic and what comes with such a special gift. Could turn into something more too if characters decide to start blabbing about it.