Answer:
proper planning and sanitation
Step-by-step explanation:
Cities grew quickly, as people moved to work and live near factories and recently created centers of industries. Then people turned into crowds having living conditions that lacked health standards. The lack of sanitation was evident, causing spread of disease and deaths resulting from choleras, pollution. The early establishments had no sanitation facilities properly suiting these needs. People usually left garbage and human wastes along the city streets , outside city buildings that led to a high disease rate. Death rate also increased with the new pandemics.
Most of the world tendency moves to continue posing the initial issues as the rural areas are in decay. Today, this continues to be a real threat, as one billion people live in urban slums, typically overcrowded, highly polluted and filled with social issues like crime. They lack the most basic services, clean water and sanitation systems.