For premature aging disorders, see Progeroid syndromes. Senescence () (from , meaning “to grow old”, from senex) or biological aging (also spelled biological ageing) is the gradual deterioration of function characteristic of most complex lifeforms, arguably found in all biological kingdoms, that on the level of the organism increases mortality after maturation. The word “senescence” can refer either to cellular senescence or to senescence of the whole organism. It is commonly believed that cellular senescence underlies organismal senescence. The science of biological aging is biogerontology. Senescence is not the inevitable fate of all organisms. Organisms of some taxonomic groups (taxa), including some animals, even experience chronological decrease in mortality, for all or part of their life cycle. On the other extreme are accelerated aging diseases, rare in humans. There is also the extremely rare and poorly understood “Syndrome X”, whereby a person remains physically and mentally an infant or child throughout one’s life. Even if environmental factors do not cause aging, they may affect it; in such a way, for example, overexposure to ultraviolet radiation accelerates skin aging. Different parts of the body may age at different rates. Two organisms of the same species can also age at different rates, so that biological aging and chronological aging are quite distinct concepts. Albeit indirectly, senescence is by far the leading cause of death (other than in the trivially accurate sense that cerebral hypoxia, i.e., lack of oxygen to the brain, is the immediate cause of all human death). Of the roughly 150,000 people who die each day across the globe, about two thirds—100,000 per day—die of age-related causes; in industrialized nations, moreover, the proportion is much higher, reaching 90%. There are a number of hypotheses as to why senescence occurs; for example, some posit it is programmed by gene expression changes, others that it is the cumulative damage caused by biological processes. Whether senescence as a biological process itself can be slowed down, halted or even reversed, is a subject of current scientific speculation and research.