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 someone has:
Severe difficulty breathing or cannot speak, cry, or feed normally
Blue, grey, or very pale lips, tongue, or skin
Long pauses in breathing
Marked chest pulling or grunting with each breath
Severe drowsiness, confusion, or difficulty waking
Signs of collapse or unresponsiveness
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 prompt medical advice if a baby, child, or adult is breathing faster than usual, wheezing, drinking much less, passing less urine, or becoming more unwell.
A baby younger than 3 months with a temperature of 38°C or 100.4°F or higher needs urgent medical assessment.
Dehydration also needs urgent assessment.
Warning signs include very few wet diapers, no urine for many hours, a very dry mouth, no tears when crying, or unusual sleepiness.
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
The article also says
People at higher risk should seek advice early if symptoms are worsening.
This includes very young or premature babies, older adults, and anyone with significant heart, lung, immune, swallowing, or muscle problems.
Every sign above is quoted from the sofidex article on RSV infection. If you are not sure, err upward — it is always reasonable to be checked.