19 Essays on Infinite Lifespans   Michael R. Rose of death. Needless to say, human mortality patterns are not necessarily a reflection of aging, on this definition. Even in Western populations during the 20th Century, there are dra- matic spikes in mortality associated with the 1918 outbreak of ‘Spanish’ influenza, World War I and World War II. [1] Surely such mortality is not to be confused with the mor- tality associated with aging? No professional biologist would imagine that there will ever be organisms that are immune to all possible causes of death or sterility. The surface of the sun is going to kill all terrestrial life that happens to find itself there without protective equipment. If there is such a thing as biological immortality, it cannot mean survival under all conceivable conditions. Instead, we can describe immortality more sensibly as a fea- ture of rates of survival or reproduction. An important side issue is whether or not fertility should be included with sur- vival in definitions of aging. For some medical professionals, the loss of fertility with age in both men and women is a clear manifestation of aging. For other professionals, it is merely incidental. If we use a definition of aging based on declining survival and fertility, we can define immortality intelligibly. If aging can  be  defined  as  the  persistent  decline  of  these  biological variables, then it makes sense to define immortality as a prop- erty of organisms that do not exhibit such declines. They may have never exhibited declining survival and reproduction, or they may have reached a point of equilibration at which fur- ther sustained declines have ceased. Having defined aging and immortality in a concrete way, we can now proceed to discuss them in a roughly empirical manner.