As regards:
“You could ingest millions of tapeworm eggs and they’d have absolutely no effect…”
I take it neurocysticercosis didn’t show up on your radar:
http://discovermagazine.com/2012/jun/03-hidden-epidemic-tapeworms-in-the-brain
http://blogs.discovermagazine.com/bodyhorrors/2012/08/19/oy-vey-the-pig-tapeworm/