Prion diseases occur in all mammals at very low concentrations, maybe one in ten million animals. It is natural that a mad cow is found at infrequent intervals. If it becomes frequent, then one can worry. And mad cow disease is not particularly infectious.
Tens of millions of Britons ate infected meat and there were less than four hundred cases of the disease, though if symptoms appear, you are one of the unlucky ones as it is always fatal. It seems that you need to ingest a lot of bad beef with multiple exposures, AND be genetically vulnerable to this disease.
It is nothing to really worry about all that much though. Your chances of dying in a car wreck are far higher.