The letter that indicates an animal has a disease named one of my colonists instead (who was caravanning at the time of notification).
After checking the colonist's health, he did indeed have the flu.
This just happened a second time, under the exact same circumstances. I don't know if there's any correlation, but again, it struck a member of a caravan:
(https://i.imgur.com/EZ4K1GP.png)
(https://i.imgur.com/722KmOT.png)
One last note:
The affected colonist who was named in the above screenshots has been on Penoxycyline without a single break for years, and has a supply of 10 with him on the caravan; plague should be impossible for him.
I have seen that too, also a colonist on caravan.