if that's your definition of biological warfare, then you need to consider to yourself that the u.s. government is then waging biological warfare against its own people.
> Biological warfare [...] is the use of biological toxins or infectious agents such as bacteria, viruses, insects, and fungi with the intent to kill or incapacitate humans, animals or plants as an act of war.
Knowingly allowing the spread of a highly contagious, lethal virus fits the definition. The only point of contention is whether the party's actions were due to incompetence or malice.
> Biological warfare [...] is the use of biological toxins or infectious agents such as bacteria, viruses, insects, and fungi with the intent to kill or incapacitate humans, animals or plants as an act of war.
Knowingly allowing the spread of a highly contagious, lethal virus fits the definition. The only point of contention is whether the party's actions were due to incompetence or malice.