Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Call emergency services if a rapidly spreading skin infection is accompanied by confusion, fainting, severe weakness, or difficulty breathing.
Fungal nail infection by itself is not normally an emergency.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek urgent medical help if the skin around the nail becomes increasingly red, hot, swollen, or painful, especially if there is pus, red streaking, or fever.
These signs may indicate a bacterial skin infection rather than uncomplicated nail fungus.
And for anyone, someone who:
has a fever with a stiff neck, a rash that does not fade under pressure, or severe headache
has a wound that is hot, spreading red, or leaking pus
3
Does any of this apply?
Book an appointment
Arrange a routine appointment if:
A nail remains thickened, discoloured, or distorted
The change is spreading to other nails
The nail is painful or makes walking difficult
Pharmacy treatment has not helped
You are considering antifungal tablets
You are unsure whether the change is fungal
You develop a new or changing dark streak in a nail
The article also says
Seek earlier medical advice for any foot or nail problem if you have diabetes, poor circulation, reduced feeling in your feet, or a weakened immune system.
Every sign above is quoted from the sofidex article on Fungal nail infection. If you are not sure, err upward — it is always reasonable to be checked.