Quoted By:
Slight grimdark version: the Joy's and Jenny's are a part of a long standing clan like an Uchiha that were naturally incorporated into modern day Healthcare. They were responsible for modern health advancement and they either practice intense inbreeding or they have wack genetics. Daughters are pressured to become nurse joys in the future and have an identity crisis often. But from the Anime we see that average people can marry a Nurse joy and have a... Nurse joy daughter?
More grimdark version is that there is an original Nurse Joy, Nurse Joy "Eve" if you will, and every joy you meet were the byproduct of eugenics to make a hyper competent nurse employed in all sectors.