As the infection progresses the nail can become brittle, with pieces breaking off or coming away from the toe or finger completely.

[9] People with onychomycosis may experience significant psychosocial problems due to the appearance of the nail, particularly when fingers – which are always visible – rather than toenails are affected.

[10] Dermatophytids are fungus-free skin lesions that sometimes form as a result of a fungus infection in another part of the body.

The causative pathogens of onychomycosis are all in the fungus kingdom and include dermatophytes, Candida (yeasts), and nondermatophytic molds.

[2] Dermatophytes are the fungi most commonly responsible for onychomycosis in the temperate western countries; while Candida and nondermatophytic molds are more frequently involved in the tropics and subtropics with a hot and humid climate.

Other dermatophytes that may be involved are T. interdigitale, Epidermophyton floccosum, Tricholosporum violaceum, Microsporum gypseum, T. tonsurans, and T. soudanense.

Scytalidium mainly affects people in the tropics, though it persists if they later move to areas of temperate climate.

Other risk factors include perspiring heavily, being in a humid or moist environment, psoriasis, wearing socks and shoes that hinder ventilation and do not absorb perspiration, going barefoot in damp public places such as swimming pools, gyms and shower rooms, having athlete's foot (tinea pedis), minor skin or nail injury, damaged nail, or other infection, and having diabetes, circulation problems, which may also lead to lower peripheral temperatures on hands and feet, or a weakened immune system.

[2] The four main tests are a potassium hydroxide smear, culture, histology examination, and polymerase chain reaction.

[3] Nail plate biopsy with periodic acid-Schiff stain appear more useful than culture or direct KOH examination.

[2] Avoiding use of antifungal therapy by mouth (e.g., terbinafine) in persons without a confirmed infection is recommended, because of the possible side effects of that treatment.

[2] Although eficonazole, P-3051 (ciclopirox 8% hydrolacquer), and tavaborole are effective at treating fungal infection of toenails, complete cure rates are low.

In conclusion the authors say that terbinafine has a relatively benign adverse effect profile, with liver damage very rare, so it makes more sense cost-wise for the dermatologist to prescribe the treatment without doing the PAS test.

(Another option would be to prescribe the treatment only if the potassium hydroxide test is positive, but it gives a false negative in about 20% of cases of fungal infection.)

On the other hand, as of 2015 the price of topical (non-oral) treatment with efinaconazole was $2307 per nail, so testing is recommended before prescribing it.

The risk is most serious for people with diabetes and with immune systems weakened by leukemia or AIDS, or medication after organ transplant.

[47] The term is from Ancient Greek ὄνυξ onyx "nail", μύκης mykēs "fungus",[48] and the suffix -ωσις ōsis "functional disease".

The basis of laser treatment is to try to heat the nail bed to these temperatures in order to disrupt fungal growth.

[2] There is also ongoing development in photodynamic therapy, which uses laser or LED light to activate photosensitisers that eradicate fungi.