Improvements in medical treatment and diagnosis mean that we're doing better than ever against some of the world's worst diseases – but scientists say those factors alone aren't enough to explain the current decline in diseases like dementia, heart disease, and colon cancer.

While it's definitely great news that these afflictions are on the wane, researchers want to better understand just what we're doing right, as it could then be used to get other types of diseases under control.

The biggest drops are happening in wealthy, developed countries like the United States, so it's possible there's some connection to be found there, researchers think.

First, the positive news: while these diseases are still around and causing significant pain and suffering, the statistics show they're dropping in frequency.

Reported death rates in colon cancer have fallen by nearly 50 percent since the mid-1980s (it's falling in all types of cancer, but nowhere near as much). That's a drop that can't be explained by improvements in screening alone.

Meanwhile, hip fractures are dropping by 15-20 percent per year, and although better drugs are playing a part, they can't fully account for what's happening. One suggestion is that as a population we're getting fatter, and larger people have stronger bones, but that's still just a hypothesis.

Incidents of dementia have been falling by 20 percent per decade, since the 1970s, according to a recent study, which looked at patients in the US, the UK, and Sweden. But there's still an "incomplete understanding" of why this has happened, the report's authors suggest – though better management of blood pressure and cholesterol are helping, because strokes from heart disease can cause dementia in some cases.

Then there's heart disease itself: numbers of deaths linked to the disease have been dropping for half a century in the US, though the rate has recently started to slow. Again, there are multiple factors involved – better treatment, better prevention, fewer smokers – but experts say there's still an unknown element at work.

There are various hypotheses for why all of these diseases are in decline, but researchers generally agree that we're still in the dark in terms of fully satisfactory explanations.

Steven Cummings, who works at the California Pacific Medical Centre Research Institute and takes a special interest in the dropping rates of major diseases, has a potential explanation of his own: something is happening with the way our cells age.

As he told Gina Kolata at The New York Times, all these diseases might be affected by small improvements in how our cells are adapting to growing older – a hypothesis he intends to explore in future research.

Of course, our bodies are evolving and changing all the time, and diseases have waned in the past for reasons that experts can't all agree upon (such as the decline of tuberculosis), so there are loose precedents for these kinds of unexplained changes in health.

Up until the late 1930s, stomach cancer was the number one cause of cancer deaths in the US, but today it accounts for just 1.8 percent of cancer deaths overall. Was it because people stopped eating smoked and salted food? Or did an increase in the use of antibiotics kill off the bacteria that cause stomach cancer in the first place? We're still not sure.

While these improvements in health are of course something to be grateful for, the research to understand just what's causing them is bound to continue until we have some answers – and hopefully then, we'll be able to speed up the decline even further, or help use what we've learned to tackle other related diseases.