The bubonic or black plague may be a rare disease in the U.S. associated with the Middle Ages and low-income regions, but in the last three decades it has started to surface in more affluent areas, a new study has found. 

A study conducted by the Marshfield Clinic Research Foundation in Wisconsin has found cases of the vector-borne disease in New Mexico, MedPage Today reported. The plague is usually attributed to poor, unsanitary living conditions, but the study has found shifts in its socio-economic and geographical distribution. 

Where human plague cases occur is linked to where people live and how people interact with their environment, noted lead researcher Anna Schotthoefer of the Marshfield Foundation. These factors may change over time, necessitating periodic reassessments of the factors that put people at risk.

Plague is caused by the fast-moving bacteria yersinia pestis, which can be spread through flea bites as well as through the air, and the latest study has found it is more common for those exposed to natural environments that support plague to contract it. 

In the past three decades researchers found that the distribution of plague in New Mexico has moved from a concentration in the rural northwest of the state to the north-central area, where the cities of Santa Fe and Albuquerque are located, MedPage Today reported. 

The study published in the July issue of Emerging Infectious Diseases reveals that 11 cases of human plague have been found in the U.S. since 1976, most of them in New Mexico. 

Plague discovered in more affluent areas in recent years was found in areas where housing developments where built in wild reservoirs of plague. The disease was likely spread by squirrels and woodrats, according to Schotthoefer.

We know that plague only exists where you have wild animals, and once a reservoir of plague is already present it is likely to persist. It isn't only about squalor; it's about where the reservoir is, Dr. Marc Siegel, an associate professor of medicine at NYU Langone Medical Center in New York City, told Health Day. 

Overall, our results confirmed the role of living in or near habitats that support maintenance of sylvatic plague as a risk factor for human Y. pestis infection, but also suggested migration of middle to upper-class families into such areas may be contributing to changes in the locations of plague cases, the researchers wrote.