CNN -- Two people in China are being treated for plague, authorities said Tuesday.It's the second time the disease, the same one that caused the Black Death, one of the deadliest pandemics in human history, has been detected in the region -- in May, a Mongolian couple died from bubonic plague after eating the raw kidney of a marmot, a local folk health remedy.The two recent patients, from the Chinese province of Inner Mongolia, were diagnosed with pneumonic plague by doctors in the Chinese capital Beijing, according to state media Xinhua They are now receiving treatment in Beijing's Chaoyang District, and authorities have implemented preventative control measures.Plague, caused by bacteria and transmitted through flea bites and infected animals, can develop in three different forms.Bubonic plague causes swollen lymph nodes, while septicemic plague infects the blood and pneumonic plague infects the lungs.Pneumonic -- the kind the Chinese patients have -- is more virulent and damaging. Left untreated, it is always fatal, according to the World Health Organization (WHO) During the Middle Ages, plague outbreaks devastated Europe, killing around 50 million people.Since then, we've invented antibiotics, which can treat most infections if they are caught early enough -- but the plague isn't gone. In fact, it's made a recent comeback.From 2010 to 2015, more than 3,248 cases were reported worldwide, including 584 deaths, according to the WHO.The three most endemic countries are the Democratic Republic of the Congo, Madagascar , and Peru.In the United States, there have been anywhere from a few to a few dozen cases of plague every year, according to the Centers for Disease Control and Prevention.In 2015, two people in Colorado died from the plague, and the year before there were eight reported cases in the state.Having caused close to 50,000 human cases during the past 20 years, the plague is now categorized by WHO as a re-emerging disease.According to the CDC, people usually get plague after being bitten by a rodent flea carrying the bacterium Yersinia pestis. Infected animals like cats and dogs can also infect their owners.The bacteria persists because low levels circulate among populations of certain rodents, the CDC says. These infected animals and their fleas serve as long-term reservoirs for the bacteria.A 2018 study suggested it's not just rats that are responsible -- the Black Death may have spread by human fleas and body lice.There is currently no effective vaccine against plague, but modern antibiotics can prevent complications and death if given quickly enough. However, a strain of bubonic plague with high-level resistance to the antibiotic streptomycin, which is usually the first-line treatment, was seen recently in Madagascar.Untreated bubonic plague can turn into pneumonic plague, which causes rapidly developing pneumonia, after bacteria spreads to the lungs.A recent report suggests that researchers are exploring a variety of approaches to develop an effective vaccine. Since different vaccine designs lead to different mechanisms of immunity, the authors conclude that combinations of different types might overcome the limitations of individual vaccines and effectively prevent a plague outbreak.Key steps for prevention of plague include eliminating nesting places for rodents around your home, sheds, garages and recreation areas by removing brush, rock piles, trash and excess firewood, according to the CDC.Report sick or dead animals to law enforcement or your local health officials, do not pick up or touch them yourself. If you absolutely must handle a sick or dead animal, wear gloves.Use insect repellent that contains DEET to prevent flea bites and treat dogs and cats for fleas regularly. Do not sleep with your pets as this increases your risk of getting plague.Finally, your pets should not hunt or roam rodent habitats, such as prairie dog colonies.