Dr. Damien Barrett:

There is no doubt that badgers play a role. They may well introduce the disease to a herd but it has spilled over to them from cattle before being spread back to cattle. When it gets into a cattle herd, we can use the analogy that this transmission is a fire lighter. To get into the big numbers of reactors we have seen and which many of the members will be familiar with, cattle-to-cattle transmission is required. There is far more transmission within species than between species. It is probably a relatively rare event but, when you are talking about the number of animals we have in the country, it happens too often in reality.