A lone chicken?

[aart]

My remaining hen appears healthy, but if I am to presume she's infected or a carrier, does that mean I can't bring in new chickens for her?

You can if you want, just understand the risks.
Many birds carry disease anyway,
if they are robust with good immune systems they can fight it off,
if not are not robust they can succumb.

 

[IdyllwildAcres]

I would either wait for your hen to die or cull her, then start over. Buttercup died of something, not old age and your remaining hen likely a carrier. Sorry you are going through this.

Gary