Let's Scare Jessica to Death (1971) is an interesting case - vintage (non-camp) hippy horror.
Jessica, played by Zohra Lampert, has just been released from a mental institution. Her husband Barton Heyman and his long-haired friend Kevin O'Connor are driving her to their new home in rural Connecticut in their bitchin' hearse. They stop in a graveyard so she can make a rubbing of a gravestone, and she sees a mysterious figure in white. But she doesn't mention it because she assumes it is a hallucination and she doesn't want to go back to the institution.
Their new house is a fog-shrouded old mansion that the locals all shun. They shun our friends as well, because they are dirty hearse-driving hippies. In the house, Jessica again sees a fleeting figure, but this time, the guys see it too. It turns out to be Gretchen Corbett, a pretty red-headed drifter who is squatting there. She promises to move on, but they invite her to stay for dinner. O'Connor seems taken with her, which Jessica likes, but so does her husband, which makes the voices in head jealous. Nonetheless, she gets the gang to invite her to stay on indefinitely.
But weird stuff is happening. The woman in white appears underwater when Jessica is swimming and tries to pull her under. Later, this figure shows her the corpse of the town's antique dealer below a dam, but it's gone when she tries to show people. It looks like she's cracking up again, and the more it looks like that, the worse the voices get.
Then she notices all the people in town with bites or scratches. And she finds a very old family portrait that seems to include Corbett. An ancestor? Or is Corbett a vampire?
The whole "Is the house haunted or is she crazy" theme comes from The Innocents, of course, but it plays very well here. The look at a group of people who left New York for the peace of country life and found trouble is bit more original. Living as a group, inviting in squatters, farming. Although they aren't organic at all - O'Connor sprays a ton of poison on the orchard. It's funny that poisoned apples aren't used as a theme at all.
Due to the title, I sort of thought that this was going to be a Diabolique situation. SPOILER - it wasn't.