Answered Dec 28, 2018
Biological weapons have existed for many years.
In the middle ages, an attacking army would use siege engines to catapult corpses of animals and humans who had died of various plague diseases, over the walls of castles and fortified cities.
Two future versions come to mind.
First is an intense, but short term strategy.
You would want something which spread very easily, and killed people quickly. However, you might want to occupy the geographic area afterwards, and so need some way to keep your own soldiers and colonists safe.
With a virus, you would need an enveloped influenza. They spread easily, but also degrade quickly when sitting on a surface, or exposed to air.
With a bacteria, you would need something with multi-antibiotic resistance genes (that you could insert, and/or select for). However, you would also need to have either a better antibiotic (to give to your own occupying people), or some way to turn off the resistance genes (with drugs, or chemicals that you spray on surfaces, etc). Drug development is time-consuming, so your opponent could be working on it simultaneously. Turning genes on/off is complicated, even in controlled lab conditions.
Second, would be a “salt the earth” strategy. Which would be easier and simpler.
This means not only killing everyone in the target area, but also never occupying it, or using it. For this, you need a spore-former. Such as Bacillus anthracis – Anthrax. Or something in the same category, but even more obnoxious. These can last for decades in a harsh desert, and then sprout and kill in human-friendly conditions.
A third possibility is indirect. Humans are dependent on animals and plants for food.
Some microbes will target animal livestock, or will target food-crop plants. You could use either bacteria or fungus, depending on your exact target and timeframe.
This isn’t a bullet that can only be shot once, at one target. It isn’t a bomb that explodes, and then you never hear it again.
Biological weapons will be quite happy to turn on their alleged makers.