The Rashbash (R' Shlomo Duran, the son of the Rashbatz, d. 1467) has a teshuvah (#360) written during an epidemic (unfortunately all too relevant now). At the time when he wrote this teshuvah, the epidemic had affected only non-Jews, so the question was whether it was likely to spread to Jews as well (which would then warrant declaring days of fasting and prayer). In this connection he quotes a passage from tomorrow's Daf Yomi (Shabbos 32a), שטנא בתרי אומי לא שליט (the Satan has no power over two nations at the same time), which would suggest that it might remain limited to the non-Jewish population. (Although he then cites other statements of Chazal that imply the contrary, and concludes that indeed a fast should be declared because of the likelihood of it spreading to the Jews too.)
He then writes:
...מ"מ הרמב"ם ובעל הטורים ז"ל ובעל השלחן ערוך לא חלקו בין גוים לישראל
Nonetheless, the Rambam, the author of the Turim, and the author of Shulchan Aruch made no distinction between [an epidemic affecting] non-Jews or Jews...
The question, then: who is this "author of Shulchan Aruch" that he's quoting? The standard Shulchan Aruch was written by R' Yosef Caro, who was born in 1488, some 20 years after the Rashbash passed away. Or is this teshuvah perhaps an interpolation from some later author?
