In Great Britain as a whole, the corresponding figures are 515 and 745. Nobody knows for sure. There are various theories about how myobacterium bovis spreads. Some say is passed from animal to animal through contact. Some say is airborne or that it lurks in the soil.
Farmers are strongly inclined to blame badgers, which harbour the same disease.
That theory first emerged in the early 1970s. The ministry of agriculture is very keen to eradicate the disease, not least because infected herds have to be destroyed and their owners compensated at market rates. The men from the ministry, together with independent advisers, are working on a five-point action plan, including a detailed study of the alleged badger factor. At least, killing around 12,500 of them, in selected areas. The ministry points out that it is only part of a much bigger and hugely complex study, and that the purpose is not to eradicate badgers, but establish what part if any they play in the transmission of the disease.