What counts as a high temperature
A body temperature of 38 °C (100.4 °F) or above is usually considered a fever. A fever is not a disease in itself but a response of the immune system, most often to an infection. How you feel does not always match the number: you can feel hot, shivery and sweaty at 37.5 °C and cope fairly well at 38.5 °C. So look at the thermometer, but also at what comes with the fever and how long it lasts.
The most common causes are viral and bacterial infections: influenza, colds, tonsillitis, ear and urinary infections. Childhood infections with fever include measles and chickenpox. Less often the cause is an inflammatory or autoimmune condition, overheating, or a reaction to a medicine or vaccine. Rarer but serious infections such as typhoid fever, meningococcal disease and pneumococcal disease often start with a sudden deterioration.
Fever in adults
In an adult with a cold or flu, a fever usually lasts a few days up to a week and gradually settles. If you are treating it at home and it is not improving, or it is getting worse, see a doctor. Treat a fever that comes with a severe headache, difficulty breathing, confusion, a stiff neck or a rash as urgent rather than waiting. If a fever lasts more than three days, or keeps returning in waves with no clear reason, book a visit with a general practitioner.
Fever in a child
Fever is very common in children, and in most cases a virus is behind it. For small babies the thresholds are stricter: in an infant under 3 months a temperature of 38 °C or above needs medical help the same day, and in a baby of 3 to 6 months, 39 °C or above. In older children, watch behaviour: a child who drinks, plays and responds is usually less worrying than one who is limp, refuses fluids or is unusually sleepy. If a child's fever lasts longer than five days, see a pediatrician. Do not give aspirin to children under 16.
A low fever that lasts for weeks
A slightly raised temperature (37.2 to 37.5 °C) that lingers for weeks is most often a leftover of an infection, but sometimes it needs checking, for example to rule out tuberculosis, hidden inflammation or a thyroid problem. It is more worrying if it comes with unexplained weight loss, night sweats, a cough lasting more than three weeks or enlarged glands (see swollen lymph nodes). If fever comes with a throat or chest problem, see also sore throat and cough.
What you can do before seeing a doctor
- Rest and drink fluids in small, regular amounts to avoid dehydration; water is best.
- Dress for comfort: do not wrap up tightly or strip completely, and skip cold-water or alcohol rubs.
- If the fever is making you very uncomfortable, a fever-reducing medicine is chosen by age and weight; for children under two, ask a pediatrician or pharmacist about the dose.
- Measure your temperature regularly and write it down: it helps the doctor see the pattern.
- Stay home from work or school until the fever settles, so you do not pass on the infection.
How a doctor looks for the cause
The doctor will ask when the fever started, how it has changed, whether you have been near sick people, travelled, been bitten or started a new medicine, and will examine the throat, ears, lymph nodes and skin and listen to the lungs. Depending on the findings, they may order a blood count, a urine test, inflammation markers, a chest X-ray, a throat swab or tests for specific infections. Antibiotics do not help a viral infection, so they are used only when there are signs of a bacterial cause.