Yosef Karo, Shulchan Aruch — Yoreh Deah 201…
“ Paragraph 1 — A woman does not leave her status of [ritual] impurity through washing in a bath, and even if all of the waters in the world went over her, she remains in her status of impurity, and one would be liable for Karet for [cohabiting with] her, until she immerses her entire body at once in the waters of a mikvah or a spring which contains 40 seah. ”
