One thing that make bioweapons truely existential risk is the possibility of multipandemic. Each bioweapon agent could not wipe out humanity because individual differences of the people. 50 per cent mortality for all humans from one agent is good estinate and it would be great catastrophe but not x-risks. If somehow 20 different agents with such mortality will be relized it would mean survaval 1 person from a million, and as they will be far from each other it most likely means human extinction.
The multipandemic could result from all out bioweapon war or be deliberately created as Doomsday weapon or result from quick development of cheap biolabs which will be simultaniosly availabel to large group of people. It could happene in next 10-20 years.
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In such cases people in government bunkers and the Amazon act as reservoirs/backstops.
The FHI's mini advent calendar: counting down through the big five existential risks. The second one is a new, exciting risk: synthetic biology.
Synthetic biology
Current understanding: medium-low
Most worrying aspect: hackers experimenting with our basic biology
Synthetic biology covers many inter-related fields, all concerned with the construction and control of new biological systems. This area has already attracted the attention of bio-hackers, experimenting with DNA and other biological systems to perform novel tasks – and gaining kudos for exotic accomplishments. The biosphere is filled with many organisms accomplishing specific tasks; combining these and controlling them could allow the construction of extremely deadly bioweapons, targeted very narrowly (at all those possessing a certain gene, for instance). Virulent virus with long incubation periods could be constructed, or common human bacteria could be hacked to perform a variety of roles in the body. And humans are not the only potential targets: whole swaths of the ecosystem could be taken down, either to gain commercial or economic advantages, for terrorist purposes, or simply by accident.
Moreover, the medical miracles promised by synthetic biology are not easily separated from the danger: the targeted control needed to, for instance, kill cancer cells, could also be used to target brain cells or the immune system. This would not be so frightening if the field implemented safety measures commensurate with the risks; but synthetic biology has been extremely lax in its precautions and culturally resistant to regulations.