Watch the video Dog infects humans with plague for first time in U.S. on Yahoo Finance . Vet Emergency & Referral Group Director Dr. Brett Levitzke on a plague-infected dog that spread the disease to people in Colorado. [Image source: http://blogs.wsj.com/ideas-market/2012/06/26/the-plague-still-stalks-the-u-s-west/ ]
When diseases are heavily dependent upon human populations and factors strongly related to human populations, like domestic pets, the standard behaviors for these diseases result in a diffusion pattern that can sometimes demonstrate preference for population centers. This particular outbreak in Colorado appears to demonstrate this change in behavior. Important to note however is that there is an ecological aspect to this cluster of plague cases that prevents the plague from easily migrating in an eastern direction. However, an increase in the role of domestic animals in forming the migration patterns could change these traditional diffuse patterns considerably.
My review of the plague, in US EMR/EHR, does demonstrate a density in the population centers of the eastern US. See https://www.youtube.com/watch?v=A78AZDxO0II