I would guess that it means something that millions of people were exposed to mad cow beef products but only a handful, relatively speaking, came down with Bovine Spongiform Encephalopathy. Does this mean that BSE is not very contagious or that only a small subset of people are susceptible or that the minimum infective dose is much greater than one or ??? Thanks

Mad cow disease in humans is caused by eating beef products contaminated with nervous system tissue from infected cattle. These are normally removed from food products but in the UK it was found that cattle were being fed the remains of older (infected) cattle which is what caused the outbreak.

Spot on Randy all of your assumptions are correct! The exposure dose of infected prions is known to be relevant as is individual susceptibility which is genetically determined by a number of alleles. The overall risk therefore of developing human bse is therefore very low at a population level. The risk for the individual is at the moment almost impossible to quantify.