No disease impact—pandemics don’t cause extinction.

As such, I believe it is important to carefully consider the probability of various proposed existential risk scenarios. In the subsequent two sections I will consider risks of engineered pandemics and unaligned artificial intelligence.

Engineered Pandemics

Extinction level agent exists

One initial consideration that must be addressed is how likely it is that any biological pathogen can even kill enough people to drive humanity to extinction. This places an upper limit on what any biotechnology could achieve, regardless of how advanced. Note that here I am referring to an agent such as a virus or bacterium that is clearly biological in nature, even if it is engineered to be more deadly than any naturally-occurring pathogen. I am not including entities that are non-biological in nature, such as artificial nanotechnology or other chemical agents. Whilst it is impossible to determine the ultimate limits of biology, one relevant point of comparison is the most deadly naturally-occurring infectious disease. To my knowledge, the highest fatality rate for any infectious biological agent that is readily transmissible between living humans is the Zaire ebolavirus, with a fatality rate of around 90%. It is unclear whether such a high fatality rate would be sustained outside of the social and climactic environment of West Africa whence the disease originated, but nevertheless we can consider this to be a plausible baseline for the most deadly known human infectious pathogen. Critically, it appears unlikely that the death of even 90% of the world population would result in the extinction of humanity. Death rates of up to 50% during the Black Death in Europe do not appear to have even come close to causing civilisational collapse in that region, while population losses of up to 90% in Mesoamerica over the course of the invasion and plagues of the 16th century did not lead to the end of civilization in those regions (though social and political disruption during these events were massive).

If we think the minimal viable human population is roughly 7,000 (which is near the upper end of the figures cited by Ord (p. 41), though rounded for simplicity), then a pathogen would need to directly or indirectly lead to the deaths of more than 99.9999% of the current world population in order to lead to human extinction. One could argue that the pathogen would only need to directly cause a much smaller number of deaths, with the remaining deaths caused by secondary disruptions such as war or famine. However to me this seems very unlikely, considering that such a devastating pathogen would significantly impair the ability of nations to wage war, and it is hard to see how warfare would affect all areas of the globe sufficiently to bring about such significant population loss. Global famine also seems unlikely, given that the greater the number of pandemic deaths, the more food stores would be available to survivors. Perhaps the most devastating scenario would be a massive global pandemic followed by a full-scale nuclear war, though it is unclear why should a nuclear exchange would follow a pandemic. One can of course devise various hypothetical scenarios, but overall it appears to me that a pathogen would have to have an extremely high fatality rate in order to have the potential to cause human extinction.

In addition to a high fatality rate, an extinction-level pathogen would also have to be sufficiently infectious such that it would be able to spread rapidly through human populations. It would need to have a long enough incubation time such that infected persons can travel and infect more people before they can be identified and quarantined. It would also need to be able to survive and propagate in a wide range of temperatures and climactic conditions. Finally, it would also need to be sufficiently dangerous to a wide range of ages and genetic populations, since any pockets of immunity would render extinction considerably less likely. Overall, it is highly unclear whether any biological agent with all these properties is even possible. In particular, pathogens which are sufficiently virulent to cause 99% or more fatality rates are likely to place such a burden on human physiology such that they would have a short incubation time, potentially rendering it easier to quarantine infected persons. Of course we do not know what is possible at the limits of biology, but given the extreme properties required of such an extinction-level pathogen, in my view it is very unlikely that such a pathogen is even possible.