The fact that average life expectancy has increased by more than 30 years during this century suggests to many experts the prospect of humans routinely living to age 150 or much longer by the end of the 21st century.

• Improvements in water supply, personal cleanliness, living conditions and immunizations had their greatest impact in the first half of the century.

• More recently, significant gains have been realized through increased awareness of the dangers of cigarettes, solar radiation, excessive alcohol consumption, environmental toxins and sedentary habits.

• In the future, genetic engineering -- including the ability to switch on and off genes affecting senescence and death, and replacing them with other bits of DNA -- will create changes so profound as to be unimaginable at this point, scientists say.

• Beyond that, there are breathtaking predictions associated with embryonic stem cell research.

But while people will certainly live longer -- with as much as one-fifth of the present population alive in 2100 -- they will not necessarily be robust. Experts warn that the future contains a large number of debilitated people. There will be as-yet unforeseeable sicknesses that will not previously have manifested themselves -- because one must live very long for them to appear.

Amid all the euphoria surrounding advancements in basic biologic research, experts caution that we should place a priority on alleviating the problems of physical frailty.

Source: Sherwin B. Nuland (Yale Medical School), "Immortality and Its Discontents," Wall Street Journal, July 2, 1999.