BTW - We empirically do not need AGI for computational/intelligence explosion.
Single virus has no chance of out computing and evading an immune system, but billions of billions of viruses can out compute and evade even human civilization as a whole.
Maybe not outsmart or outcompute, but outpower (unless you count any biological activity as computation).
Viruses have nano-level biological leverage and that's enough to wreak havoc.
The threat I'm most worried about is automated capitalism which is gradually gaining power (and intelligence, and psychological leverage) while folks are on a descending slope.
Single virus has no chance of out computing and evading an immune system, but billions of billions of viruses can out compute and evade even human civilization as a whole.