Final answer:
Chronic disease became a main focus of medicine in the 19th century, during the period of rapid urbanization. Advances in municipal planning and the implementation of measures to combat disease played a role, as did the practical applications of chemistry to medicine, such as the invention of anesthesia.
Step-by-step explanation:
The focus on chronic disease in medicine can be traced back to the 19th century. Before this time, disease was a major threat to human populations, and the link between filth and disease was understood. However, it was during the rapid urbanization of the 19th century that the problem of chronic disease became more urgent, leading to important advances in municipal planning and the implementation of measures to combat disease in some European cities. Additionally, the first practical applications of chemistry to medicine, such as the invention of anesthesia, also occurred in the 19th century.