Nail fungus (onychomycosis) is an infection that more than 35 million people in the United States suffer from. It is caused by several microorganisms (anaerobic organisms called dermatophytes) that essentially eat the keratin incorporated in your nails. Fungus is a living organism that thrives in dark, moist places – and in the case of nail fungus – under and around your toe nails or finger nails. It is that flaky, brittle, yellow or discolored thick nail that you have. The discoloration is caused by the fungus being deep inside the nails.