Final answer:
The most rapid increases in average longevity at birth occurred in the first half of the 20th century largely due to declines in infant mortality rates. Improvements in sanitation, living conditions, clean water supply, and vaccinations contributed to the decline in infant mortality and increase in average longevity.
Step-by-step explanation:
The most rapid increases in average longevity at birth occurred in the first half of the 20th century largely due to declines in infant mortality rates. In the late eighteenth and early nineteenth centuries, low wages, unsanitary living conditions, and infectious diseases caused high infant mortality rates. However, improvements in sanitation, better living and working conditions, clean water supply, and vaccinations against diseases like diphtheria, cholera, pertussis, tuberculosis, tetanus, and yellow fever contributed to the decline in infant mortality and consequently, an increase in average longevity.