Bubonic Plague that ravaged the Asian steppes and Europe slowed in winter. That's when the rats nested in holes. When the weather warmed...out came the nasties again.
Just for your convenience, the answer to this Q has been known for some decades. The difficulty in turning this particular bacterium into an effective BW agent is...that noone has been willing to attempt to aerosolise it on a WMD basis; the risk of losing a top scientist IN such attempt is/has been generally deemed unacceptable. (smile...)
So...your answer to the Q is:
________________ .
Do fill in the blank, when or if you can do. Thank you!