The average normal body temperature for adults is around 36.5°C to 37.5°C. When it rises above the normal range, it is considered a fever. Fevers are typically an indication that your body is activating responses to an underlying condition. While some causes can be easily identified, others are more difficult to diagnose. This article discusses the causes and complications of fever, including lesser-known causes of fever of unknown origin (FUO).
Defining Fever
A fever is a higher-than-normal body temperature. It is often temporary and serves as a component of the body’s immune response¹. Although fever is a symptom itself, it can feel uncomfortable, and bring about a range of other symptoms. Fevers can be categorised into different grades based on their temperature range.
- Low-Grade Fever: Harvard Health Publishing² considers a temperature between 37.3°C and 38.0°C as a low-grade fever. It is defined as a temperature slightly above normal.
- Moderate-Grade Fever: Ranging from 38.1°C to 39.0°C, a moderate-grade fever usually signals a more significant infection or illness.
- High-Grade Fever: A high-grade fever sits between 39.1°C to 41°C. Almost any infection, especially if left untreated, can cause a high-grade fever. This elevated temperature typically requires medical attention.
- Hyperpyrexia: Known as an exceptionally high fever, hyperpyrexia is a medical emergency and can be life-threatening. It is characterised by a fever exceeding 41.5°C. It can result in severe symptoms³, such as muscle spasms, rapid breathing, seizures and other potentially critical complications.
Symptoms of Fever
There are some common symptoms that accompany fevers, although they may vary according to the infection or underlying condition behind the fever. These are:
- Chills and shivering
- Sweating
- Headaches and body aches
- Fatigue and weakness
- Loss of appetite
- Flushed complexion and feeling hot
- Elevated heart rate
Common Causes of Fever
A fever can result from various causes and can be a sign of a wide range of illnesses. Nonetheless, common causes include:
- Viral Infections: Viral infections are illnesses caused by viruses. Examples include the common cold, flu (influenza), COVID-19, chickenpox and HIV.
- Bacterial Infections: Similar to viral infections, bacterial infections are caused by microbes, and can be spread to others through exposure and/or contact. However, bacterial infections can be more severe than viral infections if left unaddressed, often requiring antibiotics for effective treatment. Pneumonia, gonorrhoea and some food poisoning are bacterial infections.
- Inflammatory Diseases: Inflammatory conditions occur when the body experiences prolonged excess inflammation, which can lead to fever. Examples of such conditions include inflammatory bowel disease, lupus, rheumatoid arthritis and other autoimmune conditions
- Heat Exhaustion: Overexposure to high temperatures can lead to a dangerous loss of water and salt, causing an increase in body temperature. Hyperthermia can also be caused by medications.
- Urinary Tract Infections: Urinary tract infections (UTIs) fall under the category of bacterial infections and are a common condition affecting up to half of women at least once in their lifetime. In children, fever is one of the most noticeable signs of a UTI.
Lesser Known Causes of Fever
Sometimes, fever can result from less obvious conditions that are not as widely recognised or suspected at first glance. This can be due to factors such as subtle or short-term symptoms, or the presence of other complex symptoms that make the fever less immediately apparent. Some of them are:
- Autoimmune Disorders: Autoimmune diseases are when the body mistakenly identifies its own tissue as foreign and attacks them. A low-grade fever can be observed during flare-ups of autoimmune disorders such as lupus and rheumatoid arthritis.
- Cancer: Some cancers are known to cause “malignant fever”, though the exact reasons are not fully understood. Also called “tumour fever”, it is typically associated with the presence of cancer and may arise from cancer-related complications.
- Medications and Vaccinations: Certain drugs can disrupt the body’s temperature regulation, developing a fever. This may occur due to effects on heat dissipation, changes in metabolism, or even allergic reactions. On the other hand, vaccinations can trigger a fever as the immune system responds to the vaccine, simulating a mild infection to build up defences.
Fever of Unknown Origin
Fever of unknown (FUO) is characterised by a persistent fever lasting for 2 weeks or longer, with no identifiable cause even after thorough medical investigation. These cases are now less common due to advances in technology and enhanced laboratory diagnostic tests. However, they still occur occasionally, accounting for 5 to 15% of cases where the cause is initially undetectable⁴. Among those where a cause is eventually identified, the reasons include:
- Infections (30 to 40%): Whether caused by bacteria, viruses, fungi or parasites, the development of difficult-to-diagnose fever may be induced by deep tissue abscesses, slow progression of infection, atypical symptoms and other factors e.g. Subacute endocarditis, TB, fungal infections.
- Neoplasms (20 to 30%): Neoplasms refer to the abnormal growth of tissues, such as tumours, including cancer. Their presence can cause systemic symptoms like fever due to inflammatory responses, disruption of temperature regulation, and other complications e.g. lymphoma, lymphoproliferative disorders)
- Collagen Vascular Diseases (10 to 20%): Also known as autoimmune diseases, collagen vascular diseases cause damage to connective tissues. They can lead to fever, which can be challenging to diagnose because their symptoms often overlap with those of other conditions, making identification less straightforward.
- Miscellaneous (15 to 20%): This category encompasses a range of less common or harder-to-classify causes of fever of unknown origin, such as hypersensitivity syndrome and drug fever, hyperthyroidism, sarcoidosis, Kikuchi’s disease, and more.
- Inflammatory Conditions: Gout, osteoarthritis, chemical thrombophlebitis etc.
- Factitious Fever: This is not a medical condition but rather a simulation of fever that is intentionally produced or falsified by the patient, to gain medical attention or to get medical leave.
Complications of Fever
High fevers can be life-threatening if not treated promptly. Children under 5 years old⁵ are particularly at risk of febrile seizures, which are convulsions that occur during a fever. These seizures may cause a loss of consciousness, limb shaking, and body stiffness.
Fevers above 41°C can be especially dangerous, as they increase the risk of organ damage and failure. Even moderate fevers can be hazardous for adults with preexisting conditions like heart disease or respiratory issues, as the additional strain can worsen their condition. Inflammatory or infective causes of fever have been associated with a higher risk of heart attack and strokes in vulnerable people.
Seek Medical Advice When Necessary
Understanding the diverse causes and potential complications of fever is essential, especially when dealing with lesser-known or more complex conditions. While many fevers are the body’s instinctive response to infection and may resolve with minimal intervention like over-the-counter (OTC) medicine, others can signal more serious health issues that require prompt medical attention. If your fever is high or persists for several days, consult a healthcare professional immediately.
For more information on fevers and fevers of unknown origin, visit our page here.
References
- Fever – Symptoms and causes. Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/fever/symptoms-causes/syc-20352759. Accessed August 12, 2024.
- Fever in adults. Harvard Health. https://www.health.harvard.edu/diseases-and-conditions/treating-fever-in-adults. Accessed August 12, 2024.
- Seladi-Schulman J. Causes and Treatment for a Very High Fever (Hyperpyrexia). Healthline. https://www.healthline.com/health/hyperpyrexia. Accessed August 12, 2024.
- About Fever (Pyrexia) Of Unknown Origin (Fuo Or Puo). ID Specialists. https://idspecialists.sg/infections-treatments/fever-of-unknown-origin/. Accessed August 12, 2024.
- Febrile Seizures (for Parents) – KidsHealth. https://kidshealth.org/en/parents/febrile.html. Accessed August 12, 2024.