In 1914, three men in New Orleans examine rats to determine if they are carrying bubonic plague.Corbis

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.

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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.

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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.

The Oriental Rat Flea carries the bacterial infection that causes Bubonic Plague. An infected person typically dies within four days. Dr. Arthur Siegelman/Visuals Unlimited/Corbis

Plague, in humans, doesn't pop up a lot. Generally there are between one and seventeen cases per year in the United States, and only about a thousand in total since 1900. Anyone of any age can get contract the infection, but probability of exposure increases with outdoor activity. Lie down with small mammals, get up with plague.

People who want to minimize their possibility can go snow-camping, or camping during the coldest part of the winter -- although getting the plague may seem the more pleasurable option. Cases of plague are very rare, and antibiotics are effective, but the disease is no joke. It still carries a 11 percent overall mortality rate.

In 2012, two major cases of bubonic plague hit the news. An Oregon man attempted to dislodge a partially-eaten mouse in his pet cat's throat. The mouse was most likely infected. The cat bit the man, which would have left large open wounds for the bacteria to get into. He was treated, but lost his fingers and toes to infection before he recovered.

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In another case, a seven-year-old girl in Colorado put her sweater on the ground next to a dead squirrel. The fleas on the squirrel may have jumped to the sweater, which the girl subsequently tied around her waist. Within a day she had a high fever and dangerous septicemia, and doctors were stumped. When they questioned her mother, the woman remembered the dead squirrel, and the girl was treated with antibiotics. She recovered within the week.

Both cases point to one of the obstacles in treatment of the plague. It's incredibly well-known, but still almost unthinkable. No one seriously considers the plague these days, and a few scattered cases throughout an entire nation doesn't mean that people will recognize it when it comes.

Remembering, and mentioning, exposure to dead or dying animals is key. As for efforts to eliminate it completely - although they've been considered, it would take too much extensive trapping and testing of animals to be easily accomplished while the plague is still so rare. It looks like we'll continue getting the plague for the foreseeable future.