Death. Death is the permanent cessation of all biological functions that sustain a living organism. Phenomena which commonly bring about death include aging, predation, malnutrition, disease, suicide, homicide, starvation, dehydration, and accidents or major trauma resulting in terminal injury. In most cases, bodies of living organisms begin to decompose shortly after death. Death-particularly the death of humans-has commonly been considered a sad or unpleasant occasion, due to the affection for the being that has died and the termination of social and familial bonds with the deceased. Other concerns include fear of death, necrophobia, anxiety, sorrow, grief, emotional pain, depression, sympathy, compassion, solitude, or saudade. Many cultures and religions have the idea of an afterlife, and also hold the idea of reward or judgement and punishment for past sin. Senescence refers to a scenario when a living being is able to survive all calamities, but eventually dies due to causes relating to old age. Animal and plant cells normally reproduce and function during the whole period of natural existence, but the aging process derives from deterioration of cellular activity and ruination of regular functioning. Aptitude of cells for gradual deterioration and mortality means that cells are naturally sentenced to stable and long-term loss of living capacities, even despite continuing metabolic reactions and viability. In the United Kingdom, for example, nine out of ten of all the deaths that occur on a daily basis relates to senescence, while around the world it accounts for two-thirds of 150,000 deaths that take place daily. Almost all animals who survive external hazards to their biological functioning eventually die from biological aging, known in life sciences as senescence. Some organisms experience negligible senescence, even exhibiting biological immortality. These include the jellyfish Turritopsis dohrnii, the hydra, and the planarian. Unnatural causes of death include suicide and predation. From all causes, roughly 150,000 people die around the world each day. Of these, two thirds die directly or indirectly due to senescence, but in industrialized countries-such as the United States, the United Kingdom, and Germany-the rate approaches 90%. Physiological death is now seen as a process, more than an event: conditions once considered indicative of death are now reversible. Where in the process a dividing line is drawn between life and death depends on factors beyond the presence or absence of vital signs. In general, clinical death is neither necessary nor sufficient for a determination of legal death. A patient with working heart and lungs determined to be brain dead can be pronounced legally dead without clinical death occurring. As scientific knowledge and medicine advance, formulating a precise medical definition of death becomes more difficult. Signs of death or strong indications that a warm-blooded animal is no longer alive are: Respiratory arrest. Cardiac arrest. Brain death. Pallor mortis, paleness which happens in the 15-120 minutes after death. Algor mortis, the reduction in body temperature following death. This is generally a steady decline until matching ambient temperature. Rigor mortis, the limbs of the corpse become stiff and difficult to move or manipulate. Livor mortis, a settling of the blood in the lower portion of the body. Putrefaction, the beginning signs of decomposition. Decomposition, the reduction into simpler forms of matter, accompanied by a strong, unpleasant odor. Skeletonization, the end of decomposition, where all soft tissues have decomposed, leaving only the skeleton. Fossilization, the natural preservation of the skeletal remains formed over a very long period. Main article: Medical definition of death The concept of death is a key to human understanding of the phenomenon. There are many scientific approaches and various interpretations of the concept. Additionally, the advent of life-sustaining therapy and the numerous criteria for defining death from both a medical and legal standpoint, have made it difficult to create a single unifying definition. One of the challenges in defining death is in distinguishing it from life. As a point in time, death would seem to refer to the moment at which life ends. Determining when death has occurred is difficult, as cessation of life functions is often not simultaneous across organ systems. Such determination therefore requires drawing precise conceptual boundaries between life and death.