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. Scientists marvel at this good news, a medical mystery of the best sort. 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. [Dr. Steven R. Cummings of the California Pacific Medical Center Research Institute], intrigued by the waning of disease, has a provocative idea for further investigation. He starts with two observations: Rates of disease after disease are dropping. Even the rate of "all-cause mortality," which lumps together chronic diseases, is falling. And every one of those diseases at issue is linked to aging. Perhaps, he said, all these degenerative diseases share something in common, something inside aging cells themselves. The cellular process of aging may be changing, in humans' favor. For too long, he said, researchers have looked under the lamppost at things they can measure. "I want to look inside cells," Dr. Cummings said. Inside, there could be more clues to this happy mystery.