this patient zero would have had to be infected in another way, or have a genetic defect that caused the ribosomes to synthesise PrPSc directly, or is the PrPC able to

The initial patient likely had a mutation to the gene that produces PrPc similar to the sporadic form of CJD - it occurs at a rate of about 1 in every million people if I remember correctly.

Technically such a person would be producing the abnormal protein.

So it is likely that at some point a number of people consumed a relative/friend who had the sporadic mutation and whose body contained the abnormally shaped version.

After doing this, these people then became carriers and subsequently when they died their bodies were consumed and those people became carriers and if their bodies were consumed that spread it onto others...

This way even a very rare disease event i.e. the sporadic mutation can get amplified because of the way that these prion diseases spread.

Further due to the potentially very long incubation period in certain genotypes many cases would be unknown as the people would potentially have died of other causes before they developed Kuru.

Sorry about the incessant questions, I'm in sponge mode 8-).

No problem I find it fascinating to talk about. It is hard to say with too much certainty about these things as there is still a lot that is not known.