What causes fever?

The immune system can reset the body's temperature control while it responds to viruses or bacteria. A raised temperature is a symptom, not a diagnosis, so the cause depends on the wider pattern of symptoms, recent exposures, medicines, and medical history.

Respiratory infections, throat infections, sinus infections, stomach infections, and urinary infections can all cause fever. Some illnesses produce a rash, sore throat, cough, congestion, vomiting, diarrhoea, or pain in a particular area. Rubella and scarlet fever are examples of infections that may include fever and a rash; tuberculosis can cause a longer-lasting fever with other symptoms.

Fever can also occur with non-infectious inflammation, a reaction to a medicine, or overheating. A thermometer reading alone cannot identify the cause. The person's age, general health, temperature pattern, and accompanying symptoms matter.

Rest, drink fluids regularly, and wear light clothing in a comfortable room. Check your temperature with a thermometer and note when the fever began, how it changes, and any new symptoms. Follow the label or a clinician's instructions if you use a fever-reducing medicine, and check before giving medicines to a child.

Avoid strenuous activity and alcohol while you are unwell. Do not use leftover antibiotics: they do not treat viral infections and can delay appropriate care. A clinician can decide whether examination, a swab, urine testing, or another assessment is useful.

Seek urgent medical help for fever with trouble breathing, chest pain, confusion, a seizure, a severe headache with a stiff neck, a purple or rapidly spreading rash, fainting, or signs of severe dehydration such as very little urine. These symptoms can signal a serious illness such as sepsis and need prompt assessment.

Arrange medical advice if the fever is persistent or repeatedly returns, is worsening, follows significant heat exposure, or occurs in someone who is pregnant, has a weakened immune system, or has a serious long-term condition. Infants and young children may need a lower threshold for assessment, especially if they are unusually sleepy, difficult to settle, feeding poorly, or breathing differently.

Ask what causes are most consistent with your symptoms, whether you need testing, which medicines are suitable for you, and what changes should prompt urgent review. Mention recent travel, sick contacts, new medicines, heat exposure, rashes, and any immune or long-term health condition.