A Guide to Continuous Fever and Unknown Fever

What is a continuous fever?

A continuous fever is a body temperature greater than 37.7C (100F) that continues to persist for 24 hours or more. While there may be fluctuations in the body temperature during this time, the readings do not return to a normal range.

An acute fever lasts 4 days or less but may continue for up to 7 days in more severe infections. Chronic fevers are usually recurrent fevers that persist over a long period of time. An acute fever may be continuous for the short time span that it persists, however, in most cases the fever is episodic. A chronic fever is intermittent with recurrent episodes of an elevated body temperature lasting for a few hours and then easing only to return again within a short period of time. In contrast, a continuous fever is unrelenting.

Continued Fever Readings

  • Normal body temperature = 36.5C (97.7F) to 37.7C (100F)
  • Fever > 37.7 C (100F)
  • Low-grade fever < 38.3C  (101F)
  • Mild fever = 38.3C (101F) * to 40C (104F )
  • High fever > 40C (104F)

* A temperature above 38.3C  (101F) in an infant younger than 3 months is considered high and should be treated as a medical emergency. Refer to Baby Fever.

Causes of Continuous Fever


Most fevers are caused by an infection. A continuous fever may be indicative of a severe infection that is progressing and serious complications like sepsis may arise.


A continued fever may also indicate damage to the thermoregulatory center in the hypothalamus (central fever) that is responsible for maintaining the body temperature set point. A continued elevation in body temperature may be a result of impairment of the thermoregulatory functions that are responsible for cooling the body.


Drug-induced fever or drug-induced hypothermia that is continuous is most likely due to the chronic use of drugs for the treatment of medical conditions, abuse of illicit drugs due to addiction or unsupervised use of certain drugs like weight loss medication.

Unknown Fever

Unknown fevers may be due to a number of causes with infections accounting for the majority of the cases. An unknown fever is usually a fever that is non-specific, meaning that apart from the raised body temperature, there are no other signs, symptoms or abnormalities detected upon conducting medical tests and investigations.

A fever may be acute or chronic.

Acute Unknown Fever

In the event of an acute fever, signs and symptoms may develop shortly after the onset of the fever thereby assisting with a diagnosis. The cause may be self limiting and resolve spontaneously or may require medical treatment.

Chronic Unknown Fever

A chronic fever that is due to unknown causes is referred to as a fever of unknown origin (FUO). There are certain criteria that define a fever of unknown origin. Therefore, it is important to consult with a doctor and have the necessary tests before defining a chronic fever as a fever of unknown origin.

  • Chronic Fever
    • Persistent fever
    • Low-grade fever
    • Mild fever
    • High fever

Fever of Unknown Origin

A fever of unknown origin (FUO) is recurrent episodes of a body temperature greater than 38.3C (101F) that persists for 3 or more weeks with no cause diagnosed after 3 days of inpatient evaluation or more than two outpatient visits.

Please note that any information or feedback on this website is not intended to replace a consultation with a health care professional and will not constitute a medical diagnosis. By using this website and the comment service you agree to abide by the comment terms and conditions as outlined on this page

Ask a Doctor Online Now!
  • Joy

    I went to local library and spent a couple of hours about a month ago. Because the library was so cold that I got sick the next one or two days. Same thing happened about two more times and even going to coffee shop would make me sick because of the air-conditioning. I didn’t know I had fever at first. I just felt so much headache some pain on the right eye. The headache also involved a feeling as if some inseck walking in side of my right side of brain as these insect-walking feeling in two lines in my brain. Ever siince I have had low-grade fever which have been about a month now. I began checking the fever about a week ago and somehow, I could not really detect any fever on thermometer. I am not sure whether it’s the thermometer or me not having enough fever. When I have a fever, I also have quite severe headache as well. Time to time I cough and have swollen throat, but not all the time. For first three weeks I have been in a cycle where, after doing house chores, I got sick. I think I had fever all those days, but I didn’t realize it was fever but rather thought that I wasn’t feeling well. And about 10 days ago, I realized that I had fever and decided to take medication for it. I took took Tylenol and Advil, but mostly Advil. After 7 days of taking medication, I stopped taking medication because I thought I took the medication too long and wanted to see whether I would feel better without the help of medication by completely stopping house chores. But the following day I had ear pain, so the next day I began taking medication again and it got better and now I don’t have ear-pain, though I can hear sound, which usually happens when I feel weak. BecauseI don’t have health insurance, I have been procrastinating going to a clinic. For past three days, I have less or almost no fever especially in the morning. But I would have fever again in the afternoon and throughout the night still. For past months I always woke up in the middle of the night and for past 10 days I took medication (with food because Advil would upset my stomach). I am having hard time digesting fatty meat or greasy food, which I think that it’s due to the fever I have. I have been ill three times past 6 months. The symptoms have been similar in a way that I was ill for extended period all three times, though I don’t remember whether I had so much headache during those two past illness.

    • Dr. Chris

      Hi Joy

      Let me start by saying that there is no way to replace the expertise of your doctor as well as the necessity of a physical examination and relevant tests. Your symptoms are vague and non-specific – headache, fever, pain and indigestion after eating a fatty meal. The possibilities are endless really.

      Firstly it has to be established whether you are running an actual fever or just feeling ‘feverish’. The nature of the symptoms does warrant further testing for infections, autoimmune conditions and endocrine disorders. This means that you will have to see a doctor.

      You do not mention your age and past medical history but I would advise an HIV test (refer to First Signs of HIV). Of course, this is not the only infection or medical condition that may cause these symptoms but HIV testing is always advisable if you are sexually active.

      In addition, your gallbladder may also need to be investigated. Bottom line, you need to see your doctor as there is no other way of isolating the possible cause.

  • shivshweta

    My father age is 48 and also having diabetics. fever comes from 1 month .
    hemoglobin is 10. CBC, Cough, urine is normal. his admitted last 10 days.
    ultrasound , heart beat is normal. fever come 5-6 times in a day approx 104 c. after 10 minutes fever down approx 99 C without any help. 4 bottle of glucose daily consume. doctor unable to say what the resign behind

    • Dr. Chris

      Hi Shivshweta

      The hemoglobin is approaching a borderline anemia level. Unless he is a vegetarian and not eating properly, it does raise questions as to whether there is any blood loss. Not sure what you mean by “cough in normal”. It really depends as to when this fever started. It may be an infection (but would have been indicated in the CBC) or it could be due to other causes possibly related to complications of diabetes. 4 bottles of glucose a day??? That is not advisable for a diabetic, unless he his blood sugar levels are dipping but even then this quantity of glucose is not necessary. Start off by seeing an endocrinologist (specialist who deals with endocrine problems).

  • marthu

    hello doctor,

    im living in uk, age 26, last 9days im having fever, sometimes the body temperature goes down sometime temp goes high eyes will be paining. i took paracetamol, lemsip, beechams all in one tablets, having a very bad cough, if i started to cough it wont stop for few seconds, sometimes it’s dry cough and times i get phlegm. throat is also block i have running Norse as well, got back headache feel like phlegm is inside the head. i got fever for 8 days two week back as well.

    i went to the GP , but she is not giving tablets.

    looking for ur advice. thank you

    • Dr. Chris

      Hi Marthu

      This may just be a prolonged acute infection like the flu or common cold. However 8 days is a bit long. You should speak to your doctor about it or seek advice from another doctor. If it is persistent or recurrent even after the infection resolves, then you may have to consider further tests to try to identify less obvious infections or even autoimmune causes. Be cautious of conditions like meningitis which can arise after an acute upper respiratory tract infection and cause symptoms like pain behind the eyes and headaches.

  • Manisha Mani

    Dear Doctor,
    I am Manisha, I am suffering from persistent fever from more than one month. I went to a GP and did lots of investigation. Except my ESR and WBC all are normal. I have headache and burning eyes but no other specific sign or symptoms. The fever reaches upto 101 but gradually comes down to normal. Paracetamol and antibiotics are not helping me. Please advice.