well if you take the position that Adam was created with an immune system -- which you did feel qualified to assert, even that he had an amazingly great one to help him be immortal -- presumably that means the immune system is like, functioning and there for a reason. that reason is disease, sickness, viruses, bacteria, plagues, rot and decay.
so if you say Adam was born with an immune system, then don't you imply that death and disease were part of the unfallen creation called "very good" ?
either that or that he had one in anticipation of death and decay and deterioration through bacteria and viruses -- as tho sinless Adam on sinless Earth was created with a defense system designed to live in an world poisoned by sin.