Either make Kuru less deadly, or make Prions harder to get.

[Grayshift]

I don't mind prions having a minimum threshold to take effect because they're easy to find and/or manufacture and totally incurable. Other disease reagents should stay accessible to malicious homeopathy! They're not hard at all to cure, and it's not as if a bottle of spider eggs automatically kills forty people. Getting the reagent into a lot of people is the challenge, doing it stealthily doubly so. You've either already spent at least five or six crystals for the sleepypen/emagged hypospray plus a poison bottle, more if you're unlucky on the disease draw.

Basically: Kuru was the problem, not spider eggs/gibbis/nanomachines.