אך מעין ובור מקוה מים יהיה טהור (שמיני יא:לו)

Although one needs to be cautious not to be near or eat together with someone afflicted with a contagious illness so that he shouldn’t get infected, we see through the words of the possuk, that one who toivels – immerses himself in a Mikvah where others toivel, in order to purify himself, whether other users are healthy or not, it won’t affect him since a Mikvah can’t accept impurity or disease.