Since the sightings stopped after the bridge collapse, and one sighting was over the bridge just before the collapse, some paranormal investigators theorize that the mothman was actually some type of psychic projection.
Because of this, many researchers theorize that migraines are caused by an overabundance of naturally occurring brain substances that that cause blood vessels to swell and press on nerves in the head.
Some protesters of clay litters even theorize that bentonite can coat the intestines, preventing the absorption of nutrients and causing an overgrowth of unhealthy bacteria.
Even though it's science-fiction, the wormhole theory is a popular one that many theorize could also allow travel into other dimensions or parallel worlds.
Mitochondria are components in cells that create energy, and some theorize that mitochondria dysfunction in brain cells may lead to the symptoms of autism.