"To establish whether medical science has managed to ensure quality of life during the additional years of life now available in many populations requires a relatively complex interactive model of morbidity and mortality.  The development of such a model has been the prime objective of this paper.  For the model we describe, formulae were derived which allow a health scientist to explain the contributions, by age, of different diseases to changes in life expectancy in healthy or less than healthy states, and for a particular disease, the contribution of disease prevention, successful disease treatment (leading to recovery) and mortality prevention.  Such an analysis can be very helpful for health policy purposes.  Formulae were also developed for studying trends in life expectancy differentials in terms of changes in rates of disease prevention, disease treatment success and mortality prevention." (SUMMARY IN FRE)