Final answer:
Epidemiology's significant achievements in the 20th century, prominent in developed nations, have led to striking mortality declines due to clean water systems, public health behavior changes, and medical advancements like vaccines and antibiotics.
Step-by-step explanation:
The demographic group that had the most dramatic reduction in deaths during the 20th century is difficult to specify without additional data, but significant accomplishments in epidemiology have led to overall mortality declines across various populations. Advancements such as the development of clean water systems, changes in public health behaviors, and medical innovations like vaccines and antibiotics have played crucial roles in increasing life expectancy and reducing death rates from infectious diseases.
The improvements in public health and medical interventions have had a larger impact on life expectancy and mortality rates in developed nations, given their greater access to these improvements. However, infectious diseases continue to impact populations, particularly in underdeveloped regions such as sub-Saharan Africa, where HIV/AIDS has caused reductions in life expectancy.