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
Seek emergency help for:
Difficulty breathing, blue or grey lips, or severe chest pain
Confusion, fainting, a seizure, or difficulty waking
A severe headache with a stiff neck
Signs of shock, such as cold clammy skin, extreme weakness, or collapse
Uncontrolled bleeding or vomiting blood
Facial or throat swelling after an antibiotic
Follow the monitoring and emergency instructions given with treatment, especially if symptoms are intense or include breathing difficulty or faintness.
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
Arrange prompt medical assessment if you have repeated episodes of fever and may have been exposed to ticks, body lice, a rodent-infested building, or an area where relapsing fever occurs.
Do not wait for the fever to return before seeking care.
Contact a clinician promptly if fever develops during pregnancy after possible tick or louse exposure.
Babies, young children, older adults, and people with weakened immune systems should also be assessed early.
And for anyone, someone who:
has a fever with a stiff neck, a rash that does not fade under pressure, or severe headache
The article also says
A fever and chills shortly after the first antibiotic dose may be a Jarisch–Herxheimer reaction.
Every sign above is quoted from the sofidex article on Relapsing fever. If you are not sure, err upward — it is always reasonable to be checked.