Hayflick developed the first normal human diploid cell strains for studies on human aging and for research use throughout the world. Prior to his seminal research, all cultured cell lines were immortal and aneuploid. One of these new cell strains, developed by Hayflick and his colleague Paul Moorhead at the Wistar Institute in Philadelphia, Pennsylvania, called WI-38, has become the most widely used and highly characterized normal human cell population in the world. Hayflick had found that his normal cell strain WI-38 was capable of growing all of the then known human viruses. He hypothesized that because WI-38 was free from contaminating viruses, it could replace the then widely used primary monkey kidney cells, which contained several dangerous contaminating viruses. Indeed, WI-38 became used worldwide for human virus vaccine manufacture, to the benefit of billions of people.