I think you're "loading the deck" here, assuming without argument that material causality is the only kind, omitting reference to final, formal, efficient causes. This is, of course, appropriate to scientific research (I guess), but not really satisfactory even in areas of normal life, never mind theology. And it's a common view in our reductionist age!
I think you have it backwards. I am simply expresisng doubt at an extraordinary claim—"demonic" causes of illnesses—and I am simply asking for evidence to such an extraordinary claim.
You are turning the tables and supposing my doubt is baseless, as if acceptance of a "demonic" cause of illnesses is something deserving of a serious consideration!
In order for that to happen, one must provde more than someone's word for such a claim to be deserving of a consideration.
Extraordinary claims demand extraordinary evidence. If I state that there are pink unicorns on Jupiter, you'd be perfectly justified in demanding credible evidence to that effect and I would have no leg to stand on by accusing your doubt as being unjustified.
If one of these days scientists discover "life" on Mars, the world will demand proof, and justifiably so. Wen someone claims there can be illnesses caused by demons, it is justified to ask which demons and what illnesses are involved here.
I am waiting, patiently...