Final answer:
The plague's decline can be attributed to quarantine practices, improved economic conditions following depopulation, and the development of immunity among surviving populations.
Step-by-step explanation:
The plague eventually slowed down due to a combination of social measures, biological factors, and changes in environmental conditions. First, quarantine and travel restrictions were imposed by some cities, which also closed markets and prohibited large gatherings, including funerals. These public health measures, despite the lack of germ theory, helped to mitigate the plague's spread. Second, economic changes that reduced the vulnerability of the population, such as the depopulation caused by the plague itself, led to increased wages and better living standards for the survivors. Third, the development of local immunities in some populations and differential mortality due to varying individual health statuses meant that over time, those with a stronger immune response were more likely to survive and contribute to population resistance.