RBG
interesting dream few nights ago about that kinda thing; where cats were the spread and getting real sick and dying but not before infecting most of the nation.
it can be many things to spread an eradicated or modified disease to have larger mortality rate. Think if domestic roaming animal that everyone is trying to save it would expose dozens of people for every animal.
who?
Here is a list of known (non militarized) zooinotic diseases.
Many that just 'cause flu symptoms' not listed. Many that would decimate food supply not listed ie Foot and Mouth and Bird Flu.