Shellfish allergy is not caused by iodine, it is caused by an enzyme in shell-producing animals (including insects, and even brine shrimp in tide pools) that in turn can produce an allergic reaction if you touch and/or eat them.
Stomach ulcers are caused by a bacteria, a fairly recent medical discovery that upended decades of medical assumptions that it's primarily caused by stomach acids or stress etc.
Both of these things are surprisingly still not as well known as you'd expect within medical communities, even experts in particularly relevant fields, because there's so much literature and research that comes out every month and year.