We know pathogens that are extremely contagious, and we know pathogens that are extremely deadly. We also know toxins that are lethal at nanogram/kg doses. There's no reason to believe that a sufficiently advanced intelligence couldn't come up with a way to combine those traits.

One plausible scenario is depicted in detail in "If Anyone Builds It, Everyone Dies" (Yudkowsky & Soares 2025), so I refer you to that.