The death toll from disease will depend on which agent(s) are loosed. Every disease has a particular "standard" mortality. Of course given the malnourished condition of the Japanese this goes up, and of course the very young and very old will be at greater risk even before you throw in the malnutrition. Given the known agents unit 731 was working with I would expect a death rate averaging >50% of the infected. I would expect pretty widespread extension of the diseases.