Which part of it? The souls part, or the Joestar bloodline part? The infects souls thing is my guess. It's a disease that causes you to manifest magic ghost powers. I don't think it's a biological disease.
As for the Joestar bloodline part, that seems fairly self-evident. DIO, Joseph, Jotaro, Holly and Josuke all got Stands/fell ill at the same time. At the time DIO shot himself with an arrow.
There's no evidence though that those were naturally occurring though.
DIO HAD arrows. All of his henchmen could just be people he shot.
They never say HOW Polnareff or Iggy got Stands. They could have easily been shot, or had family members that were shot. Just because they didn't show you how they got their stands doesn't mean they just happened spontaneously.
A. It's probably not actually metal. It's probably crazy alien ghost bones or something.
B. As far as why some get Stands and some die, EVERYONE gets a Stand(Holly got her vines, one of the victims in this flashback had some kinda lightning power), just not everyone survives getting one.
As to why, why does any disease kill some people and not others? Some people are tougher than others, some people just get lucky, etc.