ABSTRACT
Rapid population ageing, or a rising percentage of older people in the population, was a 20th century phenomenon in developed countries and is now affecting most of the world’s populations, the poor as well as the rich. Countries as disparate as Australia, Iran, Thailand and Tunisia are approaching or achieving below replacement fertility levels.1 The immediate cause of world population ageing was fertility decline; this followed the reduction in infant deaths due to infectious diseases from the 19th century onwards and their substitution by deaths due to adult onset degenerative diseases in the first half of the 20th century. The infectious diarrhoeas, influenza and tuberculosis, in children and young people, were gradually replaced by cardiovascular and lung diseases at older ages. Omran,2 in 1971, referred to this shift in disease patterns as the “Epidemiologic Transition”; and he described three disease transitions occurring in developed countries up to the mid-20th century. This chapter examines population ageing and life span in relation to further disease transitions and changing causes of mortality and morbidity later
in the 20th century. A progressive delay in age of onset, and a decline in mortality from the systemic degenerative diseases (such as cardiovascular and lung diseases) was described as the fourth transition of “delayed degenerative diseases” by Olshansky and Ault in 1986.3 We are now seeing yet another substitution of mortality due to later onset neurodegenerative disorders, such as dementia and Parkinson’s disease (PD).4;5 It is predicted that the neurodegenerative disorders will gradually replace the systemic degenerative disorders as the major causes of both death and morbidity in the 21st century.
