Fever itself is a normal immune response and is usually not dangerous in adults. What matters more than the exact number is how the person looks and what other symptoms are present. Warning signs that need urgent medical care include a fever with a stiff neck, severe headache, confusion or drowsiness, a rash that does not fade when pressed with a glass, difficulty breathing, chest pain, persistent vomiting, or signs of dehydration.
Fever in certain people should always be assessed quickly: those on chemotherapy or immune-suppressing medicines, people without a working spleen, and anyone who appears very unwell or has a very high temperature that will not come down. A fever that lasts more than a few days, keeps returning, or follows recent travel to areas with malaria or other infections also needs medical review.
For an otherwise well adult, a short-lived fever with cold or flu symptoms can usually be managed with rest, fluids, and advice from a pharmacist. If red flags appear or you feel rapidly worse, seek urgent care.