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Something strange is going on in medicine. Major diseases, like colon cancer, dementia and heart disease, are waning in wealthy countries, and improved diagnosis and treatment cannot fully explain it. And this news will certainly complicate the process of reserving for a lifetime medical award.

According to the report in the New York Times, scientists marvel at this good news, a medical mystery of the best sort and one that is often overlooked as advocacy groups emphasize the toll of diseases and the need for more funds. Still, many are puzzled.

Of course, these diseases are far from gone. They still cause enormous suffering and kill millions each year. But it looks as if people in the United States and some other wealthy countries are, unexpectedly, starting to beat back the diseases of aging. The leading killers are still the leading killers – cancer, heart disease, stroke – but they are occurring later in life, and people in general are living longer in good health.

Colon cancer is the latest conundrum. While the overall cancer death rate has been declining since the early 1990s, the plunge in colon cancer deaths is especially perplexing: The rate has fallen by nearly 50 percent since its peak in the 1980s, noted Dr. H. Gilbert Welch and Dr. Douglas J. Robertson of the Geisel School of Medicine at Dartmouth and the Veterans Affairs Medical Center in White River Junction, Vt., in a recent paper.

Screening, they say, is only part of the story. “The magnitude of the changes alone suggests that other factors must be involved,” they wrote. None of the studies showing the effect of increased screening for colon cancer have indicated a 50 percent reduction in mortality, they wrote, ‘nor have trials for screening for any type of cancer.’

Then there are hip fractures, whose rates have been dropping by 15 to 20 percent a decade over the past 30 years. Although the change occurred when there were drugs to slow bone loss in people with osteoporosis, too few patients took them to account for the effect – for instance, fewer than 10 percent of women over 65 take the drugs.

Dementia rates, too, have been plunging. It took a few reports and more than a decade before many people believed it, but data from the United States and Europe are becoming hard to wave off. The latest report finds a 20 percent decline in dementia incidence per decade, starting in 1977.

A recent American study, for example, reports that the incidence among people over age 60 was 3.6 per 100 in the years 1986-1991, but in the years 2004-2008 it had fallen to 2.0 per 100 over age 60. With more older people in the population every year, there may be more cases in total, but an individual’s chance of getting dementia has gotten lower and lower.

The exemplar for declining rates is heart disease. Its death rate has been falling for so long – more than half a century – that it’s no longer news. The news now is that the rate of decline seems to have slowed recently, although it is still falling. While heart disease is still the leading cause of death in the United States, killing more than 300,000 people a year, deaths have fallen 60 percent from their peak. The usual suspects: Better treatment, better prevention with drugs like statins and drugs for blood pressure, and less smoking, are, of course, helping drive the trend. But they are not enough, heart researchers say, to account fully for the decades-long decline.