The present invention pertains to methods for determining the change in the immune repertoire of an individual suffering from disease or undergoing therapy for the treatment of a disease. The methods use the difference between the change in the level of immune cell diversity seen in an individual before, during or after a health event to determine the effect of the disease, or the effect of a treatment regime, on the individual. The difference in the level of immune cell diversity is referred to as the Delta Index and is defined as the quantitative and qualitative change (gain or loss of clones) of an individual's most dynamic immune cells over time.