Cancer, Alzheimer's disease, Parkinson's and heart disease will loom large in the next millennium.

But doctors predict big advances in treating them, including a "golden age" of new cancer drugs.

In the next 50 years, the maximum human lifespan could be 120 years, according to John Grimley Evans, professor of clinical geratology at Oxford University.

He is one of the authors of Clinical Futures, a book setting out doctors' vision of the future of medicine.

BBC News Online looks at three of the main areas covered by the book:

Cancer

Cancer is mainly a disease of old age so the increase in elderly people will mean a rise in cancer cases.

By 2020, the World Health Organisation (WHO) estimates the number of people with cancer will double to 20m.

Half of these cases will be in the developing world, which has few resources to treat them.

Professor Karol Sikora, head of the WHO's cancer programme, says diet, smoking and drinking are a risk factor for 68% of current cancer cases.

He believes the next 50 years will see an increasing emphasis on individual responsibility for preventing cancer.

Smoking prevention campaigns and changes in diet could reduce cancer by 40% by 2020, he says.

Future medicine will be dominated by diseases linked to old age

He foresees improvements in screening for genetic abnormalities that could cause cancer.

Once the human gene pattern is mapped, this could extend to screening in the womb, he says.

"As our knowledge of the human genome increases exponentially over the next decade, it is likely that selection of low cancer incidence embryos will be feasible," writes Professor Sikora.

Surgery and radiology will become more targeted and less invasive, meaning there will be fewer side effects.

Professor Sikora predicts a "golden age" for new drugs to halt the spread of cancer as well as the growing use of alternative therapies.

Heart disease

Heart disease is the biggest killer in the world and will remain so by 2020 because of the world's ageing population, according to Philip Poole-Wilson, professor of cardiology at the Imperial College School of Medicine in London.

It will also become the leading cause of disability and illness, he says.

More complex ways of predicting heart disease, which focus on a multiplicity of potential causes - from smoking to diet - will be possible, using computers.

Progress in the field of genetics will mean it will be easier to predict a family tendency towards heart disease.

Professor Poole-Wilson says surgery for angina will become less invasive and may involve thin needles or lasers.

Healthy cells could be planted in diseased hearts so tissues can regenerate.

Small pumps will be available for insertion in the heart and patients will be able to be monitored by telemedicine, he says.

Womb transplants

At the other end of the scale, there will be more focus on children who will be at a premium because they will be so outnumbered by elderly people, says Catherine Peckham of the Institute of Child Health in London.

It may be possible to transplant new cells into the heart

She thinks there will be more emphasis on child health with conditions such as diabetes, heart and lung disease being traced back to the womb.

Genetic advances will make it possible to screen for all types of disorders, which could result in parents who opt to continue with pregnancy knowing their baby is at risk of developing a disease being ostracised and forced to pay the costs.

Professor Peckham says an artificial womb and womb transplants are likely to become a reality, as are 'designer babies' - genetically engineered children - for those who can afford them.