• Trichinosis is often proposed. It's a potentially deadly and debilitating disease caused by a parasitic worm which can be contracted from undercooked pig, bear and any other animals that is not purely vegetarian. The incidence of debilitating trichinosis was probably insufficient and insufficiently immediate for this to be the reason - as demonstrated by a lack of concern in most pig eating cultures.
• Wallowing in Mud has been proposed, but pigs in a natural environment prefer clean water if they can get it and will keep themselves clean if possible, but may prefer a coating of clean mud if there are a lot of biting insects (as some humans do in the same environment).
• Economic reasons have been proposed. Raising pigs would have been an economic disaster for a dryland sheep herding people, but would have been popular among the hated rich pagan city folks down in the river valleys. Jewish leaders were anxious to differentiate their people to keep them from skipping out on them and this could have been a consideration, but probably not the main one.
• The Real Reason is known by any "less developed" rural society where people live in close proximity to free range pigs. Pigs consider human excrement to be a highly desirable gourmet treat. If they see someone going out to do #2, they're right there (and they are reported infallable judges as to whether it's going to be #1 or #2). In the indelicate words of a Southeast Asian farmer, "What we shit here they eat in the city". Some people have a real problem with this though there is no significant health consequence.

  Of course, our American grain fed hogs never have the opportunity to indulge in this treat, but I remember the plight of an anthropologist who got the "revenge" in a remote Indian village in Mexico. She had to run outside and jump over a stone wall the pigs had to go around, take care of her business and then jump back before a stampede of pigs descended upon her.

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