Usually I get a salad in the company cafeteria, but today they had prime rib. That was a no brainer.
So I got back to my desk to chow down and surf (surf and turf?), and I stumble upon the mad cow story.
So how did the cow get the condition? It might not have "caught" it at all - there's a mutation that happens every so often that causes the condition to occur independently of outside influences. In humans a corresponding condition is called Creutzfeld/Jacob disease, or CJD.
Bottom line: the cow might well be the only one affected.
A popularization of this topic, perhaps dated now, is Deadly Feasts by Richard Rhodes.