In mythology the incubus was a male spirit that came upon a woman at night and had his wicked way with her. The female counterpart was called a succubus. They were just a way of explaining sexual dreams.
In the 1930s a guy named Thorne Smith wrote a couple of novels about it but this time the ghost of Marion Kirby had it away with a shy banker named George Topper. The first was called "The Jovial Ghosts" and the second was called "Topper Takes a Trip". They were made into movies and the first one is available on YouTube. (Just search for "movie Topper".) Both books are also available for free download from Project Gutenberg.
But, as Grant Fairchild pointed out, the medium for interaction between spirits and the physical world is ectoplasm.
Thanks for an interesting question.
