Fever: What It Means

A temperature of 38°C or more usually counts as a high temperature. Infection is common, but fever can have non infectious causes. Hyperthermia is a different emergency process.

Fever is a regulated rise in temperature Fever is a regulated increase in body temperature caused by a higher thermoregulatory set point. The hypothalamus and connected brain pathways coordinate this response. Inflammatory signals can increase production of prostaglandin E2, which helps raise the set point. A fever triggering substance is called a pyrogen. Pyrogens can come from microbes or the body's inflammatory response. The body may then conserve and generate heat through skin blood vessel narrowing and shivering. Feeling cold or having chills can therefore occur while body temperature is rising. Intense shivering episodes are called rigors, but rigors do not identify the cause. Sweating often appears when the set point falls and the body releases heat. Fever is not simply the body losing temperature control. However, describing it as deliberate or purposeful can overstate what the physiology proves. Fever is different from hyperthermia Hyperthermia means body temperature rises without a higher thermoregulatory set point. Heat exposure, intense physical effort or certain drug reactions can overwhelm normal heat loss. Heatstroke is a dangerous form of hyperthermia. A very high temperature with confusion, poor coordination, a seizure or loss of consciousness requires calling 999. This distinction matters because cooling is central to heatstroke treatment. Fever and hyperthermia can look similar without the surrounding details. A reading of 38°C or more usually counts as a high temperature NHS public guidance usually defines a high temperature as 38°C or more. Normal temperature varies between people, during the day and by measurement method. A person may still feel hot, cold or shivery when a thermometer reads below 38°C. One reading should not override clear signs of serious illness. The measurement site and technique affect the result. A digital thermometer should be used according to its instructions. For young children, NHS advice recommends a digital thermometer in the armpit. Forehead strips do not provide a reliable measurement. Fever is a response, not a diagnosis Infection is a common cause of fever, but it is not the only cause. Vaccination, inflammatory disease, some medicines, tissue injury and cancer can also raise the set point. Research shows that febrile temperatures can change immune activity and the growth of some microorganisms. The overall benefit or harm varies with the illness and the person. Fever should not be treated as proof that the immune system is winning. It should also not be ignored because it may have biological effects. Antipyretic medicines lower temperature and can improve comfort. They do not identify or treat the underlying cause unless the medicine also addresses that cause. Context matters more than one number alone Clinicians assess age, general appearance, alertness, breathing, circulation, hydration and associated symptoms. They also consider immune status, pregnancy, recent treatment and travel. A moderately raised temperature in a confused or breathless person can be more concerning than a higher reading in someone otherwise recovering well. Exact temperature still matters in specific groups. A low or normal temperature does not exclude serious infection or sepsis. Older adults, very frail people and those receiving cancer treatment may not develop fever. Temperature changes after paracetamol or ibuprofen do not distinguish serious from uncomplicated illness. Clinical reassessment remains necessary when warning features are present. Babies and young children have specific thresholds A baby under three months with a temperature of 38°C or more needs urgent medical assessment. Contact NHS 111 or an urgent GP service immediately. A baby aged three to six months with a temperature of 39°C or more also needs urgent assessment. Emergency features require calling 999 instead. In children, fever duration alone does not predict whether an illness is serious. A fever lasting five days or more requires assessment, including consideration of Kawasaki disease. Kawasaki disease can cause prolonged fever with features such as red eyes, mouth changes, rash, swollen neck glands or red, swollen hands and feet. A child with a first febrile seizure needs emergency help. Call 999 if the seizure continues beyond five minutes, breathing is difficult or the child remains unusually sleepy. Treat the child's distress rather than chasing the number Offer regular fluids and continue breastfeeding. Check the child during the night and watch for reduced urine, dry mouth, sunken eyes or absent tears. Paracetamol or ibuprofen can be considered when a child appears distressed or uncomfortable. They should not be used solely to reduce the temperature number. Do not give both medicines at the same time. Do not alternate them unless a healthcare professional advises this approach. Tepid sponging is not recommended. A child should not be underdressed or wrapped in excessive clothing. Antipyretic medicines do not prevent febrile seizures. A fall in temperature after medicine does not prove that the illness is harmless. Adults also need assessment when the pattern is worsening Most short lived fevers accompany infections that improve with rest, fluids and time. Adults can use paracetamol or ibuprofen for discomfort when these medicines are suitable. Contact NHS 111 or request an urgent GP appointment when a high temperature is not improving or is getting worse. Earlier help is needed when warning symptoms develop. Persistent or recurrent fever does not have one universal diagnostic threshold. Duration is interpreted with the person's age, symptoms, exposures and overall condition. Immune suppression changes the action threshold Neutropenia means a reduced number of neutrophils, which are white blood cells important for controlling infection. Cancer treatment can sometimes cause severe neutropenia. Neutropenic sepsis is a medical emergency. A person receiving systemic anticancer treatment should follow their treatment alert card and contact the oncology emergency line immediately when advised. Many cancer services use a temperature threshold lower than 38°C. They also advise immediate contact for a temperature below 36°C or sudden illness despite a normal reading. Other forms of immune suppression also increase risk. Seek urgent professional advice for fever or new illness when your clinical team has provided specific safety instructions. Recent travel can change the differential diagnosis Malaria can cause fever, chills, headache, confusion, vomiting and other non specific symptoms. Symptoms may begin months after leaving a malaria risk area. Contact NHS 111 or request an urgent GP appointment for malaria symptoms after travel to a malaria risk country within the previous twelve months. Mention the travel details directly. Travel, animal contact, insect bites, food exposure and contact with illness can all change testing. None of these factors confirms an infection by itself. Warning features can indicate sepsis or meningitis Sepsis is a life threatening reaction to infection that can worsen quickly. Fever may be present, but a low or normal temperature does not exclude it. Call 999 for confusion, slurred speech, very fast or difficult breathing, blue, pale, grey or blotchy skin, collapse or a non blanching rash. A non blanching rash does not fade when pressed with a clear glass. It can occur with meningococcal disease, but a rash may be absent. Call 999 or go to A&E for fever with a stiff neck, severe headache, marked light sensitivity, a seizure, unusual drowsiness or reduced responsiveness. Do not wait for every symptom to appear. The person's behaviour, breathing and circulation can be more informative than the thermometer reading. Examination looks for severity and a source A clinician checks temperature, pulse, blood pressure, breathing rate, oxygen saturation, hydration and mental state. They compare behaviour and function with the person's usual baseline. The examination may include the ears, throat, chest, abdomen, skin, joints and nervous system. The focus depends on the age and associated symptoms. No single examination finding identifies the cause of fever. A normal early examination does not remove the need for follow up when concern remains. Tests answer targeted questions A full blood count measures different blood cell groups, including white cells and neutrophils. The result can support assessment but does not identify the infection source alone. CRP is an inflammatory marker. It can rise with infection and non infectious inflammation, while an early normal result does not exclude serious illness. A blood culture checks whether bacteria or fungi grow from a blood sample. A negative result does not exclude infection, and contamination can produce a misleading positive result. Urine, respiratory or other samples are selected when the clinical pattern supports them. Detecting an organism does not always prove that it caused every symptom. Imaging or a lumbar puncture may be required when a hidden source, meningitis or another specific condition is suspected. These tests are not routine for every fever. Antibiotics treat susceptible bacterial infections, not fever itself. Viral illnesses, inflammatory conditions and hyperthermia require different management. When to get help Call 999 or go to A&E for possible infection with confusion, slurred speech, severe breathing difficulty, colour change, collapse or reduced responsiveness. Call 999 for a non blanching rash, a first seizure, a seizure lasting more than five minutes or fever with stiff neck and marked light sensitivity. Call 999 for signs of heatstroke, including a very high temperature with confusion, poor coordination, a seizure, fast breathing or loss of consciousness. Do not drive yourself. Contact NHS 111 or an urgent GP service now for a baby under three months with 38°C or more. Do the same for a baby aged three to six months with 39°C or more. Seek urgent advice when a child's fever lasts five days, the child is dehydrated or they are not behaving normally. Call 999 if emergency features are present. If you are receiving cancer treatment, follow your alert card and contact the oncology emergency line immediately for the stated temperature threshold or sudden illness. Contact NHS 111 or request an urgent GP appointment for malaria like symptoms after travel to a malaria risk country within twelve months. Adults should seek urgent GP or NHS 111 advice when fever is not improving, is worsening or occurs with concerning symptoms. This lesson explains how clinicians assess fever and high temperature. It cannot identify an individual cause or replace urgent medical care.

The thermometer is one piece of evidence. Fever mechanism, age, behaviour, breathing, hydration, immune status, travel and associated symptoms determine what the reading means.