feverThe normal active human has a temperature of about 37°C. The word “about” is used advisedly, because the temperature is not an absolute value.  A woman’s temperature rises by up to half a degree after she ovulates in the middle of her cycle. Many people have temperatures up to a degree below the average with no adverse effects. The body temperature will also vary slightly depending on the time of day, food intake and the climate. All these factors must be taken into account when the notion of a normal temperature is considered.

A person’s temperature is measured using a thermometer. A thermometer may be:-

  • a graduated glass tube filled with alcohol or (less commonly these days due to its toxicity) mercury.
  • an infrared sensitive electronic probe placed in the ear.
  • a temperature sensitive electronic probe placed anywhere in or on the body that is accessible.
  • a heat sensitive strip placed on the skin. These only a very rough guide to the patient’s true internal temperature.

Someone who has a fever (raised temperature) is described as febrile or feverish.

A fever (pyrexia) is a sign that the body is fighting an infection, inflammation, or invasion by cancer or foreign tissue. A fever may be beneficial to the patient, because many germs (viruses particularly) are temperature sensitive, and are destroyed by the fever. A fever over 40°C though should be reduced by using paracetamol or aspirin and cool baths.

An infection by a bacteria (eg. pneumonia, tuberculosis, tonsillitis, ear infection, urinary infection), virus (eg. common cold, influenza, hepatitis, chickenpox, AIDS) or fungus (eg. serious fungal infections of lungs) is by far the most common cause of a fever. A viral infection usually causes a fever that comes and goes during the day, often with a sudden onset in the morning and evening, followed by a slow decline to normal over the next couple of hours. Bacterial infections tend to cause a constant fever, usually over 38.5°C. This is because bacteria reproduce like all animals, at random times, while viruses tend to reproduce all at once, so the body is subjected to a sudden doubling of the number of viruses, which stimulates the brain to increase the body temperature.

Infections can occur in any tissue or organ of the body, and other symptoms will depend upon where the infection is sited. An untreated bacterial infection will result in pus formation, and an abscess full of pus may form at any site of infection (eg. under the skin, in the lung, at the root of a tooth, in the bowel) and continue to cause a fever.

Remember that the absence of a fever does not mean the absence of infection, particularly in older people, as many elderly people do not develop a fever with infections.

Other causes of a fever include appendicitis, malaria (caused by a mosquito borne parasite), many different cancers (usually when well advanced), leukaemia, inflammation of tissue, a severe allergy reaction, rejection of a transplanted organ, autoimmune diseases (eg. rheumatoid arthritis, scleroderma, systemic lupus erythematosus), rheumatic fever, haemolytic anaemia (the body destroys its own blood cells), a blood clot in the lung (pulmonary embolus) and liver failure from cirrhosis (hardening of the liver).

Medications can sometimes cause a fever as a side effect (eg. methyldopa used for blood pressure).

Illegal drugs such as amphetamines, cocaine and LSD, are well known to cause a fever as well as tremors and sometimes convulsions.

A fever is not necessarily a bad thing. Viruses are temperature sensitive, and one of the reasons that humans develop fevers is to help kill off the viruses causing an infection.

A fever that rises and falls regularly and remains below 39°C is usually of no concern, while someone with a fever above 39°C for more than 12 hours should see a doctor within a day. A fever above 41°C is a medical emergency, and assistance should be sought immediately.

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(Last modified: 22nd Sep 2014)

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