A number of different types of fungus can cause onychomycosis including dermatophytes and Fusarium.[3] Risk factors include athlete's foot, other nail diseases, exposure to someone with the condition, peripheral vascular disease, and poor immune function.[3] The diagnosis is generally suspected based on the appearance and confirmed by laboratory testing.[2]

When the skin is injured damaged, the natural protective skin barrier is broken. Bacteria and yeasts can then invade the broken skin. Bacteria can cause a bad smell. Bacterial infection of the skin and resulting inflammation is known as cellulitis. This is especially likely to occur in individuals with diabetes, chronic leg swelling, who have had veins removed (such as for heart bypass surgery), or in the elderly. Bacterial skin infections also occur more frequently in patients with impaired immune systems.
In some cases of suspected nail fungus there is actually no fungal infection, but only nail deformity. A 2003 source gives a figure of 50%[17] whereas a more recent source claims that fungus is present in 65 to 95 percent of cases.[18] Avoiding use of oral antifungal therapy (e.g. terbinafine) in persons without a confirmed infection is a particular concern because of the possible side effects of that treatment.[17] However, according to a 2015 study, the cost in the United States of testing with the periodic acid–Schiff stain (PAS) was about $148. Even if the cheaper KOH test is used first and the PAS test is used only if the KOH test is negative, there is a good chance that the PAS will be done (because of either a true or a false negative with the KOH test). But the terbinafine treatment only cost $10 (plus an additional $43 for liver function tests). 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.[18]
Infected nails are usually thicker than normal and could be warped or oddly shaped. They can break easily. Nails with fungus might look yellow. Sometimes a white dot shows up on the nail and then gets bigger. When fungus builds up under your nail, it can loosen and even separate the nail from the bed. The fungus can also spread to the skin around your nail.