Can reused monomer become contaminated and cause or spread fungus?
No, the organic solvents in the solution will destroy any disease-causing germs in the product. It would really have to be a monster germ that could survive exposure to those types of chemicals. But remember, when the organic solvents have dried out, such as after the acrylic nail has cured, the germ-killing power of the monomer is gone. -- David Dryer
Monomer does not contain water, so neither bacteria nor fungi can survive in it. There’s no need to worry about this as a potential source of infections. Both nail enhancement brushes and monomer liquid are “self-disinfecting.” -- Doug Schoon