Up until the past century, why was mortality so high in infancy and toddlerhood?

Question

Quizplus · Accepted Answer

Infectious diseases were a major cause of high mortality rates in infants and toddlers before the twentieth century. Vaccinations, antibiotics, and advancements in hygiene have helped decrease the prevalence of many infectious diseases that once claimed the lives of young children. Accidents, physical abuse, and malnutrition can also contribute to mortality in young children, but infectious diseases were the primary concern before modern medicine.

Up Until the Past Century, Why Was Mortality So High