I think the reference is to the well-accepted cases of intentional (along with likely unintentional) introduction of smallpox to Native American populations, and at least some amount of bubonic-plague-era disposal of bodies near to enemy camps with the intent to spread it.
I don’t think it’s been used much in the modern era—the populations worth attacking are generally advanced enough to require more direct and selective means. I expect this will continue for quite some time—it’ll pick up in terrorism a bit, but organized forces with significant civilian populations to protect will realize that such weapons are too likely to escape the enemy.
There’s probably a middle ground of “semi-infectious” weapons, which can be controlled easily, but not without doing a bunch of damage before it’s understood. Or that infect, for instance, only a specific strain or growing condition of wheat.