Living in Europe during the 14th century was not a fun experience. A nasty pestilence was in progress, and it killed about a third of the population. This disease was known alternately as the bubonic plague, black death, or black plague. This affliction has been known of for centuries, and in the days before an effective treatment was developed, it carried the potential to spread rapidly through any city, village, or region. In most cases, so many people would perish that there were often not enough survivors to bury the ones who weren't so lucky.
It wasn't until 1894 when two different doctors named Alexandre Yersin and Shibasaburo Kitasato respectively noted bipolar straining organisms in the lymph nodes and organs of victims. Yersin found the connection from infected rats, and the organism was named Yersinia Pestis, or simply Y Pestis. A few years later, in 1898, another doctor, Paul Louis-Simond, found that this disease was also carried by way of fleas that had burrowed themselves into the rats. Surprisingly, the genetic material of the Y Pestis organism is pretty much the same as it was when a third of Europe was wiped out hundreds of years ago. Fortunately today, the disease is quite rare. In the US, around 10-20 people a year contract the disease. Worldwide estimates are between 1000-3000 annually. Nevertheless, this isn't something you want to catch, for about 14% of people infected with the plague will die as a result, and this can increase up to a 50-90% mortality rate if left untreated without antibiotics.
Here are the details as to how so many humans once contracted this awful condition:
Yersinia Pestis is a bacterial organism. It causes three different variations of plague: bubonic plague (the most common), septicemic plague, and pneumonic plague. It is likely that during the 1300s, all three of these plagues wreaked havoc on the European population. This wasn't exactly a sanitary time to live, to say the least. Since this Y Pestis bacteria is found primarily in rats, which happened to thrive in much of Europe at the time, one can only imagine how easily people acquired these germs. Besides rats, Y Pestis can be found in dogs, cats, fleas, prairie dogs, lice, chipmunks, squirrels, and mice.
In most cases, all it takes for a person to become infected is to be bitten either by such an animal that carries this organism or more likely by a flea that has feasted on that animal's contaminated blood. In rare instances, a tainted piece of clothing or other article from an infected person can come into contact with another person through an open cut or other pathway and infect one as well. Once infected, a person can have symptoms such as a headache, fever, chills, weakness, and swollen lymph glands. The medical science of those days was quite primitive to begin with, and those symptoms were and still remain characteristic of many other different ailments. Unfortunately, in the case of bubonic plague, the bacteria multiply rapidly in the bloodstream, and in the absence of antibiotics in the 14th century, most people that became infected simply succumbed in short order. Those who didn't contract the disease were simply lucky enough not to get bitten by an animal that carried the organism.
By far, the reason bubonic plague has become rare in this day and age is simple: Rat populations are now largely controlled, particularly in developed countries. However, the risk remains increased in underdeveloped areas where such rodents are still more prevalent.