Open source discussion of the topic in the strategic literature over the years have mentioned smallpox as a good starting point for conventional research of the old type. As far as purely theoretical discussions, many comments have been made over the years that to produce a maximally dangerous weapon, a starting point could be a strain of influenza, for its ease of transmission,vigorousness, and adaptibility, married, through genetic engineering to something like one of the hemorrhagic fevers, such as Ebola. Others have replied that, yup, this would be just dandy if the goal was to destroy all human life on earth. I think that's what you're looking for. Other lines of theory dealt with ideas like disabling but not essentially fatal infections designed to impair the ability of a country to defend itself without actually causing mass deaths. Such as, for instance, a highly infectious and fairly debilitating strain of influenza, which would greatly degrade the ability of an army to get out of bed or stray far from the latrine, which would tend to make fighting a trifle more difficult.