The United States is one of the many countries around the world that technically still suffers from what was once called the Black Death. Although we're not keeling over like medieval peasants, there are regular cases of bubonic plague that spring up every year in the American southwest. Occasionally, they lead to deaths. More often, they lead to people scratching their heads as they read the newspaper and wondering aloud, "How do we still have the plague?" It's all San Francisco's fault.
The city was ringing in the year 1900 and things looked bright. San Francisco was both a local hub of industry and a port to ships coming in from the far east. Each of those ships had to pass a health inspection before they docked, of course, but both the passengers and the local businesses pressured the health inspectors to get it out of the way as quickly as possible. They did this even after cases of plague, and mini-epidemics, broke out in China, and then in Hawaii.
It was not a surprise to health officials when the first case of plague was reported in Chinatown, but they were surprised by the opposition they faced in even saying the word "plague." Over the next few years, state and local organizations worked against federal health officials, fearing that any reports of plague would damage trade and tourism. When the 1906 earthquake hit, and the rats took over the rubble of the city, the deaths came so fast and thick that there was no denying it anymore. Still, it took years of work before the plague was quelled. By that time, it had started showing up in local squirrels.
Bubonic plague is not a virus, but a bacterial infection. Yersinia pestis lives in fleas, which leave traces of it in the area that they bite. It works its way into the body and multiplies, traveling through the lymphatic system. The swellings that appear at the groin and under the armpits are the painfully swollen lymph nodes. Bobonic plague kills within four days, at which point the fleas desert the body and go to the next victim, taking their bacteria with them.
An infected flea doesn't necessarily mean an infected host. Different fleas have different eating techniques, different hosts scratch (driving the bacteria into the wound) or don't scratch, and not all hosts act as ideal carriers for the bacteria. But plague in the wildlife won't stay in the wildlife for long.
There are certain environmental factors that affect the spread of the disease. More large predators means less teaming colonies of small rodents. Both hosts and fleas hibernate during cold winter months. Anyplace where small burrowing animals thrive in hot weather is ideal. So while the plague can go anywhere on the gradually-migrating backs of rats, it's most likely to flourish in a warm climate with lots of small mammals.