First it spreads to the community... Then to a passerby... then to another community and another and so on. The thing with TB of any sort is that you may have it for months or even years before you know about it and by then you may have passed it along to countless people. All it takes is one person leaving the community or one person passing through the community and you have a very deadly situation on your hands.
It is unfortunate that this particular bull became infected but the bull's slaughter was a necessary evil.