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The sporadic form of CJD has an extremely low incidence, about one case per million person-years, all over the world.
The etiology of sporadic CJD remains unknown, and there is concern about the possibility that some unrecognized factors may enhance the risk of developing CJD. However, no evidence was found for any environmental risk factor, so far.
CJD has been accidentally transmitted by iatrogenic routes: the incubation time is long, and highly variable between individuals: a mean incubation time of 9 to 10 years, with a 95th percentile of the distribution of 15 to 16 years, has been reported in French growth hormone recipients who were homozygotes at codon 129 of the prion protein gene (Huillard d'Aignaux J et al. Neurol 1999;53:1197-201).