You can now see an excellent visualization of global catastrophic risks estimates produced in the Ragnarök series here.

No single disease currently exists that combines the worst-case levels of transmissibility, lethality, resistance to therapies, and global reach. But we know that the worst-case attributes can be realized independently. For example, some diseases exhibit nearly a 100% case fatality ratio in the absence of treatment, such as rabies or septicemic plague. The 1918 flu has a track record of spreading to virtually every human community worldwide. Chickenpox and HSV-1, can reportedly reach over 95% of a given population.

An informal survey at the 2008 Oxford Global Catastrophic Risk Conference asked participants to estimate the chance that disasters of different types would occur before 2100. Participants had a median risk estimate of 0.05% that a natural pandemic would lead to human extinction by 2100, and a median risk estimate of 2% that an “engineered” pandemic would lead to extinction by 2100.

Moreover, previous literature has found that casualty numbers from terrorism and warfare follow a power law distribution, including terrorism from WMDs. Millett and Snyder-Beattie have performed a naive power law extrapolation to estimate the chance of an existential biological disaster:

Past studies have estimated this ratio for terrorism using biological and chemical weapons to be about 0.5 for 1 order of magnitude, meaning that an attack that kills $10^x$ people is about 3 times less likely ($10^{0.5}$) than an attack that kills $10x^{x-1}$ people (a concrete example is that attacks with more than 1,000 casualties, such as the Aum Shinrikyo attacks, will be about 30 times less probable than an attack that kills a single individual). Extrapolating the power law out, we find that the probability that an attack kills more than 5 billion will be $(5 billion)^{–0.5}$ or 0.000014. Assuming 1 attack per year (extrapolated on the current rate of bio-attacks) and assuming that only 10% of such attacks that kill more than 5 billion eventually lead to extinction (due to the breakdown of society, or other knock-on effects), we get an annual existential risk of 0.0000014 (or $1.4 × 10^{-6}$).

In the first part of the Ragnarök Question Series, we asked the question If a global catastrophe occurs, will it be due to biotechnology or bioengineered organisms? Now it is asked,

Given that a biological global catastrophe occurs that results in the reduction of global population of at least 10% by 2100, will the global population decline more than 95% relative to the pre-catastrophe population?

The question resolves positive if such a global biological catastrophe does occur, and the global population is less than 95% of the pre-catastrophe population. The question resolves ambiguous if a global biological catastrophe that claims at least 10% (in any period of 5 years or less) does not occur. The question resolves negative if a global biological catastrophe failure-mode induced global catastrophe occurs that claims at least 10% (in any period of 5 years or less) but the post-catastrophe population remains above 5%.

A biological catastrophe is here defined as a catastrophe resulting from the deployment biotechnologies or bioengineered organisms (including viruses) that claims at least 10% in any period of 5 years or less before 2100. Moreover, the catastrophe must be generally believed very unlikely in a counterfactual world with little or no biotechnological interventions but otherwise similar to ours.