I think it's a little bit more nuanced - if you think of something like the Hesperia incident, where a German ship breached a japanese quarantine (1897), or e.g. the introduction of spanish flu to Samoa - you can see it doesn't really take a devious plot to knowingly introduce a plague to a vulnerable population. It just takes the ordinary colonial mix of racism, callousness, and utter disregard for human life.
There are also things like the australian smallpox epedemic, which is a little suspect, given that australia is isolated enough that the incubation and recovery period is greater than the time it takes to travel there. Obviously, people doing stuff like this don't tend to leave a load of written records, but I think it's consistent with the mentality and capabilities of people at the time to engage in purposeful biological warfare.
There are also things like the australian smallpox epedemic, which is a little suspect, given that australia is isolated enough that the incubation and recovery period is greater than the time it takes to travel there. Obviously, people doing stuff like this don't tend to leave a load of written records, but I think it's consistent with the mentality and capabilities of people at the time to engage in purposeful biological warfare.