Being in a woodsy area, it is pretty possible that you might have brought in something on your shoes or clothing. I have heard of people bringing SDA and Sendai viruses in on their shoes(comes from squirrel feces I believe).

If you haven't brought in any outside rats, then Sendai or SDA is very possible. What were their symptoms? Just poofing up and looking poorly in general? How long did ago did the symptoms start and how long did the disease progress until you saw deaths from it? Did they have swollen lymph nodes or rattling in their chests?