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.