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

Rashi says that water connected to the ground (to a well) never becomes טמא – impure.

Although one always needs to be cautious not to share food or come in contact with an ill person who is contagious, it is evidenced that in a place where people toival (submerge) in a mikvah, whether healthy or not, it doesn’t affect another as a mikvah doesn’t absorb any impurities or illnesses.