Perspectives and needs for health in the 21st century: 20th-century paradigms in 21st-century science.

Countries in both the developed and developing world are expected to experience an unprecedented growth in the number of their elderly and decline in the number of their youth. This extraordinarily rapid aging of the world's population is a phenomenon without precedence in human history. Additionally, there is an increasing gap between the rich and poor both among and within nations. Both trends will not only constitute the primary health issues of the first decades of the 21st century but will also profoundly alter the world's economic, political, and social landscapes.