Fever
Symptoms, Causes & Treatment
Everything you need to know about fever: what causes it, when it is serious, and how to manage it safely in adults and children.
What Is a Fever?
A fever, also known as pyrexia, is a temporary rise in body temperature above the normal range. In adults, a fever is generally defined as a body temperature of 38°C (100.4°F) or above. Normal body temperature varies between individuals and throughout the day – typically ranging from 36.1°C to 37.2°C – but a reading consistently at or above 38°C is considered a fever in clinical practice.
Fever is not a disease in itself but a physiological response – a sign that the body's immune system is actively responding to something. In the majority of cases that something is an infection, either viral or bacterial. The elevation in body temperature is triggered by chemical messengers called pyrogens – released by immune cells in response to infection, inflammation, or other stimuli – which reset the hypothalamus (the brain's thermostat) to a higher temperature setpoint.
A fever is broadly considered a beneficial immune response. The elevated temperature creates an environment less hospitable to many pathogens, enhances certain immune functions, and signals that the body is mounting a defence. However, very high fevers – particularly above 39.5°C in adults or 38°C in infants under 3 months – require prompt medical assessment, as can fevers that are prolonged, recurrent, or accompanied by concerning symptoms.
What Are the Symptoms of a Fever?
The symptoms of fever vary depending on the height of the temperature, the underlying cause, and the individual’s age and general health. Recognising the difference between a manageable fever and one that requires urgent attention is important.
- Body temperature above 38°C (100.4°F)
- Feeling hot, flushed, or sweating
- Shivering or chills
- Headache
- Muscle aches and general malaise
- Loss of appetite
- Feeling thirsty or dehydrated
- Fatigue and weakness
- Temperature above 39.5°C or persistent high fever not responding to medication
- Severe shaking chills (rigors)
- Confusion, disorientation, or altered consciousness
- Stiff neck with fever and light sensitivity
- Non-blanching rash (does not fade when pressed)
- Difficulty breathing or chest pain alongside fever
- Fever lasting more than five days in adults or three days in children
- Febrile seizure in a child
Seek immediate medical attention if fever is accompanied by a non-blanching rash, neck stiffness, or severe headache (possible meningitis); confusion or altered consciousness; difficulty breathing or chest pain; a temperature that is extremely high (above 40°C) or extremely low (below 36°C); or any fever in a baby under three months old. These may indicate a serious or life-threatening condition.
What Causes a Fever?
Fever is a symptom with a wide range of causes. In most cases it signals infection, but it can also result from inflammation, medication reactions, or other medical conditions. Identifying the underlying cause is essential to guide appropriate treatment.
Viral infections — including influenza, COVID-19, the common cold, and norovirus — are the most frequent cause of fever in adults and children. The immune system raises body temperature as a defence mechanism to slow viral replication and enhance immune cell activity.
Bacterial infections such as urinary tract infections, pneumonia, tonsillitis, skin infections, and sepsis can trigger significant fever. Unlike viral illnesses, bacterial infections may require antibiotic treatment, making clinical assessment important.
Fever is a normal and expected response to many routine vaccinations. Post-vaccination fever typically develops within 12–24 hours, is low-grade, and resolves within one to two days without treatment.
Inflammatory conditions including rheumatoid arthritis, lupus, inflammatory bowel disease, and other autoimmune disorders can cause recurrent or prolonged fever as a result of immune system dysregulation rather than infection.
Some medications — including certain antibiotics, anticonvulsants, and blood pressure drugs — can cause drug fever, a febrile reaction that resolves on withdrawal of the offending agent. This is an important consideration when fever develops in someone on regular medication.
Fever of Unknown Origin (FUO) is defined as a temperature above 38.3°C on multiple occasions over three or more weeks without a diagnosed cause despite initial investigation. It requires systematic specialist evaluation to identify infection, malignancy, or inflammatory disease.
Key Risk Factors
How Is a Fever Diagnosed?
The diagnosis of fever begins with accurate temperature measurement and a thorough clinical assessment. The goal is not simply to confirm the fever but to identify its underlying cause — particularly to distinguish between viral infections that will resolve on their own and bacterial infections requiring antibiotic treatment.
How Is a Fever Treated?
Treatment of fever focuses on two goals: relieving discomfort with antipyretic medication and, where appropriate, treating the underlying cause. Most fevers caused by viral illness resolve with supportive care alone. Bacterial infections require antibiotic treatment prescribed by a clinician.
Supportive Measures
Regardless of the cause, staying well hydrated is the single most important supportive measure during a fever. Fever increases fluid losses through sweating and rapid breathing; adults should aim for at least two to three litres of fluid per day, while children should be offered regular small amounts of fluid. Light clothing and a comfortable room temperature help prevent overheating. Rest is important — avoid returning to work, school, or strenuous activity until the fever has resolved for at least 24 hours.
Fever of Unknown Origin (FUO)
Fever of Unknown Origin is defined as a temperature above 38.3°C recorded on multiple occasions over a period of three or more weeks without an identified cause despite initial clinical and laboratory investigation. FUO requires systematic specialist evaluation. The three most common categories of underlying cause are infection (most frequently tuberculosis or occult bacterial infection), malignancy (particularly lymphoma), and non-infectious inflammatory disease (such as adult-onset Still’s disease or giant cell arteritis). A GP referral for specialist assessment is appropriate when initial investigations have not yielded a diagnosis.
When Should You Seek Medical Advice for a Fever?
Any fever in a baby under three months old · Fever with a non-blanching rash, severe headache, and neck stiffness (possible meningitis) · Fever with confusion, extreme drowsiness, or loss of consciousness · Fever with difficulty breathing or chest pain · Signs of sepsis: very high or very low temperature, rapid heart rate, rapid breathing, mottled skin, or feeling critically unwell · A febrile seizure lasting more than five minutes.
Fever in a child aged three to six months · Fever lasting more than five days in an adult or more than three days in a child over six months · Fever not responding to paracetamol or ibuprofen · You are pregnant, immunocompromised, or have a chronic health condition · Fever developed after returning from travel abroad · You have recently had surgery or an invasive medical procedure · You are concerned the infection may be bacterial and may require antibiotics.
How Can You Prevent Fever?
Because fever is most commonly caused by infection, the best prevention strategies focus on reducing exposure to and transmission of pathogens, alongside supporting a robust immune response.
Staying up to date with recommended vaccinations — including annual flu vaccination, COVID-19 boosters, and childhood immunisation schedules — is the most effective way to prevent many of the infections that cause fever.
Regular handwashing with soap and water for at least 20 seconds removes pathogens before they can enter the body. This is especially important after using the toilet, before eating, and after contact with someone who is unwell.
Staying well hydrated supports immune function and helps the body regulate temperature. When unwell with a fever, increasing fluid intake is essential to compensate for losses through sweating and rapid breathing.
Avoiding close contact with people who are unwell, not sharing utensils or drinking vessels, and ventilating indoor spaces reduces exposure to the respiratory viruses and bacteria that most commonly cause fever.
Regular physical activity, a balanced diet rich in fruit and vegetables, adequate sleep, and avoiding smoking all strengthen the immune system’s capacity to fight off infection before it takes hold.
If you are travelling to regions with endemic tropical diseases such as malaria, dengue, or typhoid, consult a travel health clinician well in advance. Appropriate vaccinations, antimalarial medication, and insect bite precautions can prevent serious febrile illness.
Speak to a Clinician About Fever
If you or someone you care for has a fever and you are unsure whether it requires treatment, an online consultation with a licensed clinician can provide rapid, expert reassurance. Through The GP Service, you can speak to a clinician in minutes — from home. If a prescription is clinically appropriate, it will be sent to your pharmacy the same day.
Expert clinical advice, when you need it.
Fever FAQs
A fever is generally defined as a body temperature of 38°C (100.4°F) or above. A temperature between 37.5°C and 38°C is sometimes referred to as a low-grade fever. The method of measurement matters: rectal temperature is most accurate in infants, while oral or tympanic (ear) thermometers are reliable in older children and adults. Forehead strip thermometers are less accurate and not recommended for clinical assessment.
A fever is the body’s natural immune response to infection. Raising the core temperature makes the environment less hospitable for viruses and bacteria and activates immune cells more effectively. For most otherwise healthy adults and older children, a moderate fever is not dangerous and does not need to be suppressed unless it is causing significant discomfort. However, in infants, immunocompromised individuals, and those with certain medical conditions, fever requires prompt assessment regardless of height.
Any fever in a baby under three months requires same-day or emergency medical assessment, as infants this young have immature immune systems and can deteriorate rapidly. In children aged three to six months, a temperature above 38°C warrants urgent clinical review. In children over six months, a fever that is accompanied by a rash, difficulty breathing, persistent vomiting, signs of dehydration, or that lasts more than five days also requires prompt assessment.
Febrile seizures are convulsions triggered by a rapid rise in body temperature in young children, typically between the ages of six months and five years. They are frightening to witness but are usually brief (less than five minutes), self-limiting, and do not cause brain damage. If a child has a febrile seizure, lay them on their side, do not restrain them, time the seizure, and call 999 if it lasts more than five minutes or if this is their first seizure. All children who have had a febrile seizure should be assessed by a clinician.
Sepsis is a life-threatening emergency that occurs when the body’s response to infection causes widespread organ damage. Warning signs include a very high or very low temperature, rapid heart rate, rapid breathing, confusion or disorientation, mottled or pale skin, and feeling extremely unwell disproportionate to the apparent illness. If you suspect sepsis — particularly in someone with a known infection who suddenly deteriorates — call 999 immediately. Time is critical.
Fever after returning from abroad — particularly from tropical or subtropical regions — must always be taken seriously and assessed promptly. Malaria in particular can cause life-threatening illness and may not present until days or weeks after leaving an endemic area. Other travel-related causes include typhoid, dengue, and other tropical infections. Always tell your clinician about recent travel when seeking assessment for fever.
Paracetamol and ibuprofen are both effective antipyretics. Paracetamol is generally preferred as a first-line option as it is suitable for most people, including during pregnancy and in those who cannot tolerate NSAIDs. Ibuprofen has the added benefit of reducing inflammation and may be more effective for fever associated with inflammatory conditions. The two can be alternated in children to maintain more consistent temperature control, though this should be discussed with a clinician. Neither should be used for prolonged periods without clinical assessment.
Fever itself does not require antibiotic treatment — antibiotics only work against bacterial infections, and most fevers are caused by viruses. A clinician will assess whether the cause of your fever is bacterial before prescribing antibiotics. Taking antibiotics unnecessarily does not speed recovery, can cause side effects, and contributes to antibiotic resistance. If a bacterial infection is confirmed or strongly suspected, appropriate antibiotics will be prescribed promptly.