A mass-casualty biological or chemical attack occurs in which AI's material role is confirmed
The index says: very unlikely (1%). Graded on that date, right or wrong, in public.
Why the index thinks so
Every frontier lab now treats its top models as able to help with known bio/chem weapons and blocks such requests; Anthropic reports disrupting attempts. Mass-casualty attacks remain rare and hard to execute regardless of AI.
What would move it
Lab threat reports describing bio misuse; public-health attribution of an outbreak or attack.