There are tons of wacko conspiracy theories on the internet that I haven't had direct contact with, that's no difficult feat. There is just so much crazy out there - nobody can possibly exposed to all of it. It's the crazies and extremists you are exposed to that I find interesting.
Around the Midwest (and maybe other places) there is a group that goes preaching on college campus, Campus Ministries is their name. They'll tell you, with a straight face, that oral sex causes cancer. One of them, Brother Jed, used to tell us that accepting Jesus into his life, while eating at Burger King, instantly cured his crab infestation.
Sometimes people are just crackpots, unable to put cause and effect together. Sometimes they're just really startlingly ignorant. But sometimes, they're also really desperate. I know someone, who otherwise understands science, who believed the anti-vaccination camp for a while. She's got a child with autism and this was early in the theory, long before Jenny McCarthy threw her celebrity status behind it. She has intellectually come around but, sure enough, was reluctant to have her youngest child immunized and drew the process out longer than is recommended by pediatricians. Her daughter is, I believe, fully immunized for her age now, but I'd bet it's her lingering and desperate fear of what caused her older child's autism that was the obstacle and not her other rationalizations. I think that she and McCarthy (and many others) were so desperate to find a reason for their children's issues that they latched onto whatever threads of theories they could in the literature - and clung to them, in some degree, even when officially discredited.