Plague reached the United States around 1900 on rat-infested steamships. Early epidemics occurred in port cities, but the bacterium later became established among wild rodents and fleas in the West. That ecological shift explains the geography of modern US cases.
From urban ports to rural foci
The last urban plague epidemic in the United States occurred in Los Angeles in 1924–1925. Modern infections are usually scattered rural cases associated with western natural foci rather than sustained city transmission.
CDC identifies northern New Mexico, northern Arizona and southern Colorado as one principal region, with another in California, southern Oregon and far western Nevada. Exact risk varies within those broad areas and changes with local animal activity.
What exposure looks like today
People may be exposed through infected flea bites or contact with infected animals. Pets can bring infected fleas into human environments, and cats can themselves become ill. Public-health investigations may examine rodents, fleas and places a patient visited, but a single human case does not mean a whole state is experiencing an outbreak.
Why surveillance includes animals
Unusual rodent die-offs can be a warning signal because susceptible animal populations may decline during increased transmission. Veterinary reports and flea testing add context before or alongside human cases.
The western US story shows why plague is best understood as an ecological disease with occasional human spillover, not only as a historical urban epidemic.