By the year 2050, 3 million of us will be over 85, compared with only 500,000 in 1981. "Death rates before middle age have come down by about a half in the last 30 years, which is huge," says Richard Peto, professor of medical statistics and epidemiology at Oxford University. "Overall the pattern of mortality is likely to get better rather than worse."
Death rates from cancer, heart disease and stroke are all falling, except in a few selected age groups. Smoking-related deaths among older women, for example, are still rising as the effects of their having taken up the weed during and immediately after the Second World War are felt. But overall the trend is downwards. Moreover, the incidence of certain other disorders, such as appendicitis and stomach ulcers, which are not killers but cause much misery, is also falling.
What no one can predict, however, is whether by living to a greater age, we will experience longer periods of dependency and infirmity at the end of our lives, or whether this period will remain much the same but simply occur later. Robert Maxwell, secretary of the King's Fund, an independent health-care think- tank, says: "Optimistically, greater longevity will mean that the problems of ultimate decline and death will just be postponed. But we cannot assume that. In the over-85 age group, the proportion of people still living in their own homes, while still high, declines sharply."
A pessimist might infer that the best health strategy for the future will be to avoid a miserable old age by dying relatively young, in which case there is little point in looking after one's health. Such reasoning is flawed. In fact, most of the current evidence suggests that, the healthier you are, the shorter will be the eventual period of dependancy with which your life will probably end.
In some respects, unfortunately, staying healthy may become harder, especially for the less affluent, since experts predict a widening of the health gap between rich and poor. "By and large," says Mr Maxwell, "the messages about smoking, diet and exercise are getting through to the better educated and more prosperous, who are, to some extent, less pressurised, and not to the poorer sections of society." In addition, Britons of all classes could face new outbreaks of infectious diseases, just as they have faced the challenge of Aids in the past 15 years. Some microbes are developing resistance to existing drugs, and global warming could allow insect vectors of disease to survive in places which were previously too cold for them.
"Nature will never allow a microbiological vacuum," says Alasdair Geddes, professor of infection at the University of Birmingham. "It is a natural trend for new diseases to appear and for old ones to re-emerge. Tuberculosis, for example, which everyone thought was disappearing, has re-surfaced. The organism responsible for it is becoming resistant to the currently available drugs. And if the world experiences global warming, as the scientists predict, we in this country could have to deal with tropical diseases, such as malaria."
In the US, doctors have recently come across cases of dengue fever, imported by infected mosquitoes arriving in the US from South East Asia through shipments of old car tyres. And global warming could also mean an increased death rate among the elderly during heat waves, while the depletion of the ozone layer could result in a rise in skin cancer and cataracts.
This is not to say that overall levels of health will decline: there will, after all, almost certainly be progress in medicine as well. But it would be complacent to assume that we will inevitably win the age-old war between man and microbe. A more realistic hope would be that the current cycle of battle and truce will continue.
New whizz-bang medical advances will continue to appear, of course, but it is not clear to what extent they will affect general health - or whether we will be able to afford them. As far as advances that probably will affect the average person are concerned, John Wickham, consultant urological surgeon at Guy's Hospital, London, thinks that minimally invasive therapy, or keyhole surgery, will become the norm in the next century. "We shall look back in horror at the idea that we used to open people up and put our hands inside them," he says. He predicts that, as this trend takes off, we shall move from having large hospitals with small car parks, to small hospitals with large car parks. Certainly, the rate at which hospital beds are closing in most large cities seems to bear this out.
Other developments could be more problematic. On the one hand, new genetic tests, both for adults and babies in the womb, could make certain diseases, such as thalassaemia and sickle cell disease, problems of the past, while adults who discover that they carry the genes for disorders such as familial breast and colon cancer will be able to take preventive measures to avoid developing life-threatening disease, including regular screening tests or even, in extreme cases, prophylactic surgery. On the other hand, such developments are likely to pose heavy ethical and personal dilemmas both for doctors and for patients - whose role in decision-making will surely increase.
Transplant surgery could become more common as scientists breed animals that are genetically compatible with man. Transplant surgeon Sir Roy Calne, from Addenbrooke's Hospital, Cambridge, has conjured up the idea of everyone having a "self-pig", a specially bred animal which will be his or her immunological twin in porcine clothing. If you need a heart or kidney transplant, it will provide you with one, without any of the usual problems of rejection.
The limiting factor is not technology but cost. Mr Maxwell points out: "The things that doctors and surgeons would like to do will continue to grow faster than our capacity to pay for them. We will see many more dilemmas like the recent case of child B, in which a health authority refused to pay for a child's leukaemia treatment. New modern technologies often make great differences to countries' health care systems. They do not necessarily greatly affect health itself."