In adults, the temperature alone is a poor guide to how sick you are. A moderate fever with severe symptoms can be more concerning than a high fever in someone who otherwise feels reasonably well. As a rough guide, persistent readings above 39.5-40C, or a fever that climbs despite fever-reducing medicine, warrant medical assessment.
Go to emergency care regardless of the number if there is a stiff neck, a non-blanching rash, confusion, difficulty breathing, chest pain, repeated vomiting, a seizure, or if the person becomes hard to wake. These can signal serious infection such as sepsis or meningitis.
Thresholds are lower and stricter for babies, young children, pregnant people, and those with chronic illness or weakened immunity. If you are unsure, contact a doctor or a telehealth clinician who can help you decide. Informational only.