The Epidemiological Transition
As industrialization and urbanization increased, many—especially the urban poor—began dying at younger ages. The main killer was tuberculosis, followed by influenza, pneumonia, typhus, and other infectious diseases. Except for tubercu- losis, these diseases are all considered acute diseases—that is, diseases that strike suddenly and disappear quickly, sometimes killing their victims, sometimes causing only mild illness. By the late 19th century, however, deaths during infancy and childhood and deaths at all ages from infectious disease began to decline rapidly. Between 1900 and 1930, life expectancy—the average number of years individ- uals can expect to live—increased from 47 to 60 years for whites and from 33 to 48 years for African Americans.
As infectious diseases declined in importance, chronic diseases gained impor- tance. Chronic diseases are those that typically last several years or more such as muscular dystrophy and asthma. During this period, heart disease, stroke, arthritis, and diabetes became major causes of illness and disability. Increasingly, too, these conditions shifted from primarily diseases of the affluent (formerly the only peo- ple likely to enjoy long lives) to disproportionately diseases of the poor.
The shift from a society characterized by infectious and parasitic diseases and low life expectancy to one characterized by degenerative and chronic diseases and high life expectancy is referred to as the epidemiological transition. This transition seems to occur around the world once a nation’s mean per capita income reaches a threshold level (in 2017 dollars) of around $11,900.
