Fever

Fever is a controlled upward shift of the hypothalamic set point driven by pyrogens released during infection or inflammation. That distinction matters: it is not overheating, and the body is actively defending the higher temperature, which is why shivering and chills come first. In adults, 38.0C (100.4F) is the conventional threshold. The height of the fever correlates poorly with how serious the cause is — a 39.5C temperature from influenza is usually less dangerous than a 38.2C temperature in someone who looks grey, confused or breathless. Clinicians assess the person, not the thermometer. Most fevers in otherwise healthy adults are viral and self-limiting over three to five days. Paracetamol and ibuprofen make people feel better but do not shorten illness or reduce complications, and there is no evidence that suppressing fever helps the immune response. Fluids matter, because insensible losses rise roughly 10% for each degree above normal. Vitamin C and zinc have been studied for the common cold rather than for fever specifically: zinc lozenges started within 24 hours may shorten cold duration modestly, while vitamin C shows no benefit for treatment once symptoms begin. Nothing supplemental treats fever itself. The real clinical task is identifying who has a bacterial infection, sepsis, or a non-infectious cause such as drug fever, thromboembolism or malignancy — and that requires examination, not self-management.

• •Temperature above 38C (100.4F)
• •Chills and shivering as the set point rises
• •Sweating as the fever breaks
• •Muscle aches and headache
• •Loss of appetite and thirst
• •Fatigue and weakness

• •Viral infection (influenza, COVID-19, common cold)
• •Bacterial infection (urinary, chest, skin, abdominal)
• •Post-vaccination immune response
• •Drug fever from antibiotics or antiepileptics
• •Inflammatory and autoimmune disease
• •Venous thromboembolism
• •Malignancy, notably lymphoma

Pyrogenic cytokines including IL-1, IL-6 and TNF-alpha raise the hypothalamic thermoregulatory set point via prostaglandin E2. The body then defends the new set point by vasoconstriction and shivering. Any process that generates these cytokines — infection, tissue injury, drug reaction, autoimmune activity or tumour — can produce fever.