Why did many medieval European cities become overcrowded and unsanitary?

Medieval European cities became overcrowded and unsanitary primarily due to the physical constraints imposed by defensive fortifications and the rapid influx of people from the countryside.12

As urban populations grew, cities were often enclosed within walls to provide security. When these cities reached their capacity, the resulting constriction led to severe overcrowding, which blocked out essential air and light. Because most streets were narrow footpaths rather than thoroughfares, they were ill-equipped to handle the density of inhabitants.12

Sanitation suffered significantly because these cities lacked effective waste management systems. In many towns, residents simply threw waste into the streets or gutters, where it accumulated. While some larger cities eventually employed scavengers to cart refuse to dumps outside the walls, smaller towns continued these hazardous practices for centuries. Furthermore, because people lived in a rural manner—often keeping animals near their homes—and relied on cesspools that frequently overflowed, the environment became a breeding ground for disease. These conditions, combined with the lack of advanced drainage or sewage infrastructure, created an environment that was fundamentally detrimental to public health.3452