Here is a creepy parasitoid in wasps that might provide fodder for a Halloween EP adventure:
http://www.wired.com/wiredscience/2011/10/wasp-parasite/
Imagine a bio- or nanoengineered creature that infects people, turning them into haughty loners with an urge to travel. They seek out each other, the male creatures impregnating the female creatures, who then use their hosts to seek out new habitats where they can spread their larvae. The only trace is the emptied bodies of past hosts...
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