Hans W. Frei was born on 29 April 1922 in Breslau, Germany. Both his parents were physicians: his father, Wilhelm Siegmund, was a venereologist who invented the “Frei test” for certain venereal diseases, and his mother, Magda (née Frankfurther) was a pediatrician. They were thoroughly secularized Jews, but, in a practice common at the time, they had their children baptized as Lutherans in hopes of avoiding problems with anti-Semitism. In later years, Frei used to remark with a smile, “And in my case the baptism took!”
From The Dictionary of Modern American Philosophers in Oxford Reference.