Globally and yearly, people are infected with the bubonic plague, some even die from it. Through time, the infection's intelligence increased while antibiotics' effectiveness decreased; this happens to every virus. This forces medicine to change, advance. The Black Death was a turning point in history-where the bubonic plague spread across the world with tremendous ferocity- and old treatments were found ineffective, forcing medicine to change forever.
Pasteurella pestis was the root cause of The Black Death. It's spread by rodents and transmitted to humans by parasitic fleas that live on rodents. At the time of the black death there were unsanitary conditions, making it easy for the virus to be spread amongst people rapidly. It can only be spread by a bite from a flea, so physical contact spread the fleas among the people. The disposal of dead bodies was an issue, they were not properly disposed but put into plague pits which was highly unsanitary (pictured below).
Pasteurella pestis originally came from a harmless pathogen in the burial grounds in london.
More causes of the spread of the virus, then and now, is travel. The disease went worldwide through trade routes, passed from ship to dock continuously by rats.