It would be hard to spread the plague by sending those infected with it into an area. First off, it is spread by fleas, who live on rats, and unless an infected person is bitten by a flea which then bytes an uninfected person. Flea infestations are not that rare in the US, but getting a bunch of them to bite specifically people and then bite other specific people would be tough. People who have the plague are pretty sick and probably not around long enough to share their misery with a lot of others. It was a major problem in Europe because of the living conditions. Those conditions are not prevalent in most of America today.
I am not an expert on the subject and make no claim to be. It’s possible they have found a way to spread it without the rats that carried the fleas, but if that’s the case then all bets are off.
you need to do more reading about pneumonic plague, which is not spread by fleas, but among infected people, person-to-person, airborne