"I'm sorry, but you'll have to put down Fido because some others dogs who never even came into contact with yours had ________ disease"
Can you imagine the outrage?
If Fido had a virus that was as virulent -- read "uncontrollable and highly lethal and contagious" -- as virulent Newcastle, and Fido's illness threatened Spot and Rover and Blackie and dozens and hundreds of others who may never smell their butts but may play in the same dog park, then you'd have a comparable situation.