1691-1692: The town of Salem experiences some weird phenomenons: people hallucinating, experiencing severe cramps, itching feelings just under the skin,...

Since there is no medical explanation for these phenomenons, the people of Salem have to find another scape-goat. Influenced by Catholic Church, they go for the only logical explanation: The Devil himself must be involved. Soon, they find about 150 people that 'qualify' to be witches.

They have two options: either confess, and be hung; or not confess and commit a crime against God (for lying) which was ever worse...

1976: Linda Caporael claims to have found an explanation on why those 'victims' were suffering from the described symptons. In short, they were on LSD.
Now, LSD is a synthetic drug and wasn't around back in 1691.
Here's Linda's hypothesis: the Salem-people's main food-source was grain. The ground they cultivated it on: swamp.
In 1943, Albert Hoffman was researching ergotized grain (grain infested by a specific fungus; the fungus literally takes over the grain). While making an extract, he spoiled some of the stuff on his hand. He started to hallucinate and suffer about the same symptons as the 'Salem-victims'. Ergotized grain contains alkaloids, nerve-toxins. The same alkaloids are what gives LSD its hallucination-enducing effects.
Caporael was the first one to link the two events. Her theory: the people in Salem suffering from those symptoms weren't ill, they were intoxicated, drugged. I won't go into detail about the proof she found, but it sounds pretty convincing to me (The witch-haunts in Europe concentrated on places where grain was harvested,...). Although her 'evidence' was discounted by lots of prominent scientists, I think she's made some very good points...

So next time someone needs an exorsist: send him to Betty Ford...