Our design is incompatible with indefinite survival. In almost all multi-cellular organisms, some parts simply can’t be replaced. We have a complement of specialised (post-mitotic) cells that have very limited or no capacity to divide. These include the neurons of the brain, the beating muscle of the heart and the insulin-producing cells of the pancreas. They cannot be replaced, which is why the effects of ageing may be more important and more obvious in these cells and the functions that they serve. Ageing, Injuries, Catching diseases and decreasing of the population of helpful bacteria in our body, etc are the major factors for our limited lifespan. Thus, we can conclude that Death is Inevitable! But this is not the case for all the living organisms. Unicellular organisms are considered to be biologically immortal. They are not found to have natural death. As they grow old, they usually undergo mitosis or amitosis (cell division) by which they reproduce to form two or more offsprings. So, we see that the whole organism itself gets divided and thereby begins a new life. So, there is no way we can say that the organism has died. Therefore we can't say anything on the lifespan of unicellular organisms and we can conclude that they are immortal. In other words, we know that every organism (including a cell) has it's own life span,but because the unicellular organisms reproduce through asexual mode and divide through amitosis and involve a single nuclei cytoplasm and genetic material, the offspring produced is literally a Clone of it's earlier body as it does not loose any feature of the previous body. Therefore they are sometimes considered immortal. Source: Quora and Encyclopedia.