A few things to touch on as a response to the earlier person's comment. You just have to pay attention to what is going on in the film.
(I guess they are spoilers)
The red stuff under David's mouth? Poison ivy, the wife says not to scratch it or else "it will spread".
David goes "insane" because the stranger is telling HIM to get out of the house, which probably proves David's theory of an affair happening between the wife and stranger; he runs after the man.
David does not lose him in the woods, he simply hits the stranger a couple of times and leaves it to his wife to pick up the pieces.
Only the wife eats the mushroom.
I must say, after that one point with the wife and stranger, I began to feel disappointed. But the ending made up for the entire film.
And for that and the very last scene... this is one of my favorite movies ever. I should have put it together earlier, but let myself get sidetracked. I was really surprised, honestly.
This film is interesting, to say the least. But if you are not watching this for the performances that the actors give, I'd say you better let this one go, because that is all that keeps this certain film together.