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CJD is the most prominent of the handful of diseases, all fatal, that result when prions, proteins of uncertain function that are abundant in the brain, misfold into an infective form that spreads widely.

They suggested that individuals exposed to contaminated c-hGH, who did not succumb to CJD and lived longer, might eventually develop Alzheimer's disease.

The latest findings in Nature Medicine suggest Alzheimer's-related amyloid protein might be spread accidentally during medical and surgical procedures in the same way as CJD.

Aguzzi declined to comment on the French CJD cases, but told Science his lab never handles human or bovine prions for research purposes, only for diagnostics.

The system has identified 36 “definite and probable” cases of CJD in New Brunswick since 1998.