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 fever occurs with:
Severe trouble breathing, blue or grey skin, or collapse
New confusion, extreme difficulty waking, or loss of consciousness
A seizure, especially a first seizure or one that is continuing
A stiff neck with severe headache or marked sensitivity to light
A rash that does not fade when pressed with a clear glass
Very hot skin after heat exposure, especially with confusion
And for anyone, someone who:
has had a seizure that will not stop, or does not wake up afterwards
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 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, same-day medical advice for a baby younger than 3 months with a rectal temperature of 38°C (100.4°F) or higher.
Get same-day advice if you have a weakened immune system, are receiving chemotherapy, recently had major surgery, or have a fever plan from your care team.
Follow that plan if it uses a lower threshold.
Contact a healthcare professional promptly for signs of dehydration, including very little urine, a very dry mouth, no tears, or inability to keep fluids down.
And for anyone, someone who:
has a fever with a stiff neck, a rash that does not fade under pressure, or severe headache
3
Does any of this apply?
Book an appointment
Book a routine appointment if the fever lasts more than about three days, keeps returning, worsens after initial improvement, or occurs with unexplained weight loss, night sweats, persistent pain, or another concerning symptom.
Children younger than 6 months may need earlier assessment.
Every sign above is quoted from the sofidex article on Fever. If you are not sure, err upward — it is always reasonable to be checked.