The more I think about it, the more I suspect it'll be completely by accident. Some AI designed drug will pass all trials, get approved, be used for years or decades only to find out that some bit of it kind of acts like a generic prion that affects all life. Oh, and that bit also passes right through you, too, so by the time they figure it out, the pseudo-prion is already out there in the wild, infecting fish and other aquatic creatures. And before long the ecosystems of the world's oceans and lakes collapse. Meanwhile land animals also start dying off due to their drinking supply being polluted by it, so a full scale ecological collapse begins. As the pseudoprion sticks around indefinitely, every attempt by nature to evolve new life ends because of it. Eventually it gets buried by all the detritus of time and new life does once again rise only to have humanity's ticking time bomb waiting in the ground for something to dig it up and start the whole cycle all over again...