Which of the following threats poses the greatest threat to human health and security in the near future and why?
The deliberate or accidental release of a biological warfare agent by a state power, the deliberate use of BW agents by nonstate actors in an act of terrorism, or the emergence of a dangerous new form of communicable disease.
The greatest of these threats is the emergence of a new natural and communicable disease. All we have to think about to realize the truth of this is something like the flu pandemic of 1918.
All states have an incentive to refrain from using biological warfare. Using such weapons would surely backfire on the state that used them. They would be condemned by most other states and possibly attacked militarily. Non-state actors will have a very hard time getting possession of weaponized biological agents.
While a truly lethal new disease is not very likely to arise, it is possible for that to happen and it could be terrible if it did. While our medical knowledge has advanced tremendously since 1918, that pandemic killed millions. Our more recent experiences with things like avian flu and SARS imply that we would not necessarily be able to prevent the spread of a new communicable disease.
For these reasons, the natural threat is the most dangerous to us in the near future.