Ugh, Nemsis again... 
Blood (red blood cells, contain a single Iron atom within the complex chain of proteins)
How to try and describe this... hmm.
Iron is the star killer. Iron is the first known element to take more energy to create than you get.
Err, the fairy live in a the realm of the nevernever... or whatever you want to consister it. The realm, by its very nature... dislikes reality, or items/things that are "heavy or real". Ether, the inmaterial. As we read through the whole of the dresden universe... several "heavy" items affect the fae. Harry talk's about using Uranium (depleted) as well.

Chances are that the infection is something that would take some amount of time...

Sith wouldn't respond to Harry's summons (after an hour or so after the fight), chances are he was being restrain'd and a ritual was being preformed.
Aka... something that took time.
If it was quick, then Sith shouldv'e responded in short order to Harry's summons.

The blood is just iron, or perhaps the impruities of a moral creature in the realms of ideas. Conflicted ideas come from people, blood comes from people...