My understanding is that it doesn't quite work as easy as all that. There is a tradeoff between how easily something is going to spread, and its mortality rates. Spanish Flu killed 15% of those infected, went worldwide, killed 5% of global population. A big deal, but not 90%.
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Even Black Death "only" killed 1/3rd of Europe. Again, huge numbers. But not extinction events by themselves. High-mortality diseases like Ebola hard to spread to huge numbers — they burn themselves out.
Not saying you couldn't imagine a future in which we engineered things to really be extinction-causes — but that isn't what the Soviets had in the 1970s, at least not from the accounts I've read. They had slightly tweaked plague, etc. Not nice. But not going to kill off globe.
