[@BlackPanther] I wasn't going to look too closely at the medical details, but all tetanus needs is an anaerobic environment -- thus, a place where metal is oxidizing and eating up all the oxygen molecules is an excellent spot.  But there /are/ other ways, as you said.  I looked at it only for the connection to iron, otherwise it's not a particularly good choice.

If you want another example of cross-species disease, try rabies.  That's a pretty common one.  Various forms of flu jump species, or at least mix it up with their other counterparts before jumping back worse than ever.  There's also the matter of parasites, though that can get a bit squicky.  *shiver*  The human body itself can create issues -- there are microbes in our gut and various other places that can cause problems if out of whack.  Appendicitis and tonsillitis are two alternative possibilities.  

Colds are rarely serious.  Flu, whether you speak strictly of influenza or colloquially of something like pneumonia, is more serious.  Come to think of it, if Lyra is from a hundred years ago, and a young child, she wouldn't have any immunizations that most children nowadays do.  ...Very few fey-caught mortals would.

Curse it, now I want to start an epidemic.
Max: Please don't.
Hush you.