Ultra Fungus Fix fungus (also called onychomycosis) is made up of very small organisms that can infect fingernails and toenails. These organisms are called dermatophytes. Fungi are parasitic plant organisms, such as moulds and mildew. These parasitic organisms lack chlorophyll and therefore do not require sunlight for growth. Nail fungus is very common. More than 35 million people in the US have it under their nails.