A MATTER OF SCALE. “Humans as individuals are mortal but humans as a population are immortal. That concept applies equally well to all animals and most plants. We frequently see it stated that bacteria and protozoa are immortal. That is not so. Like human populations, unicellular populations are immortal and like individual humans unicellular organisms are mortal.”
http://protein.bio.msu.ru/biokhimiya/contents/v62/full/62111380.html