Most unicellular organisms specially those which undergoes binary fission are immortal because they reproduce by cell division. The parent body as a whole constitutes the reproductive unit and disappears when its division into daughter individuals is completed. There is no remains of parent body cell and thus, parent cannot be said to have died. Infact, after binary fission, the parent continues living as two daughter individuals.

 

In multicellular organisms reproduction occurs in specialised organs involving specialised cells. The whole body of parent dies due to ageing and senescence.