Bio War
Plague is endemic to the Western U.S.
It does not necessarily have to be plague and it doesn't have to be the Western U.S.. The Japanese were working on a variety of biowarfare agents including smallpox. We know they launched about 6,000 balloon bombs and over half of them reached the U.S. Some as far away as Michigan! It would not have taken a major leap for some of these bombs to contain biowar agents.
In fact..and here I will admit I may be entering ASB territory, how do we know that some of those that didn't make it or were never found (and that is not out of the realm of possibility according to some who have studied the Japanese balloon bombs)and may still be in some remote area in the U.S. didn't carry biowarfare.
In addition The Red, in reference to the "potentially catastrophic" response of the U.S., we are talking about a military [Japan] that had already thoroughly aroused the wrath of the U.S. by its atrocities, that had shown itself fully capable of launching kamikaze attacks, that was willing to commit itself to a suicidal defense of every strip of land it held, and had shown itself perfectly willing to use all forms of biological warfare in China. Given all of this and the fact that Japan was being bombed mercilessly by 1944 through 1945 and that even
with the atomic bombings of Hiroshima and Nagasaki
and the invasion of the Soviet Union of Manchuria the Japanese Army still wanted to fight to the death, I would have a hard time thinking of anything the Japanese military
wouldn't be capable of at this time.