It was able to quickly spread across all Europe (and probably north Africa too) though the Mediterranean trade network. A lack of hygiene and previous starvation also worsened the situation. Moreover, it’s wasn’t only the bubonic plague, there was also its pneumonic form, which is much more contagious and deadly.
Well, coronavirus have also been present throughout much of human history and infects millions of people every year. But the pandemic of 2020 have been quite unusual with a more aggressive and deadly variant. This is also the case with bubonic plague. There was a very deadly variant spreading through human populations leaving only the half of the population who became immune or who isolated themselves well enough but then the plague were mostly gone. A lot of people were immune, the deadly variants of the bubonic plague were being out-competed by the less deadly variants and people became very wary about signs of outbreaks and changed their behavior. This happened in 541 and then again in 1346 and with smaller outbreaks recorded in 1629, 1772 and 1855 onwards. The final straw for bubonic plague was that we started constantly practicing good hygiene, use good soaps, isolate ourselves from sick people, etc. which means that the bacterial infections have a very hard time spreading. Viruses are far better at spreading so we need even better techniques to deal with them including vaccines and even better hygiene.
Thats when the surface proteins change enough (in a drastic way) that the antibodies in circulation are incapable of binding to the new surface protein.
Genetic shift and antigenic drift are two other, similar yet different, events that can explain it. But antigenic shift is the most dramatic and usually the cause of epidemics in already existent infectious diseases.
Primarily because of population pressures.
By 1346 Europe was leaving the medieval warm age, a period in time which had resulted in warmer summers, larger harvests and a population growth as a result. As a result of population growth more marginal soils had been taken into use. Now that harvests were dropping (because of falling temperatures and degrading farmland quality) people were working harder for their harvests and getting less food. Also, the conditions for peasants were terrible (with the nobility squeezing their peasants pretty hard in most of europe).
High population density of poorly fed individuals (and less able to resist disease as a result) means that europe was primed for an epidemic…and being one of the first major epidemics since CE 746 (ie, several hundred years before) meant that people weren’t prepared for it in any way or form. Lots of people with the genes that meant that their immunesystem was more vulnerable to the plague (because it hadn’t been selected against for 10+ generations) and society itself was unprepared.
During the following plaguewaves people were much more observant to signs of disease, authorities more prepared, people were more well fed (since the thinning of the workforce had led to better conditions for the peasants) and people were the descendants of people who had survived the plague (so probably people whose genes allowed for a good immune response). So much lower R0 numbers (how many people each new sick individual is expected to infect).
Distrust in science plus unchecked power of the Church. Guess there is no modern equivalent…….