They don't know, they think its water or air born, which, given the filthy state of the forest at the moment, is wonderful. It has started in the same area of the forest as last time, which is quite a way away from me but later in the outbreak, a dog, who had only been walked locally to me, died of it.
They think they know the bacteria that causes it but haven't found any trace of it in any of the dead animals and can't say where the bacteria might have come from. If it is a bacteria, maybe it can be carried on car tyres or dirty wellies....