What High Fever Causes
A high fever—generally a body temperature above 39°C (102.2°F)—is the body's way of signaling that something is wrong. Understanding the underlying causes helps parents and caregivers decide when to seek medical help and how to manage the symptom safely at home.
Defining a High Fever
While a normal body temperature ranges from 36.5°C to 37.5°C (97.7°F to 99.5°F), a high fever is usually considered any temperature above 39°C (102.2°F). In children, the threshold can be slightly lower, especially for infants under 3 months.
Common Causes of High Fever
1. Viral Infections
Viruses are the most frequent culprits. Common viral illnesses that can raise body temperature include:
- Influenza (flu)
- Respiratory syncytial virus (RSV)
- Parvovirus B19
- Enteroviruses (e.g., coxsackie)
- COVID‑19
2. Bacterial Infections
Bacterial pathogens can trigger a higher fever than many viral infections. Typical bacterial causes are:
- Pneumonia caused by Streptococcus pneumoniae or Haemophilus influenzae
- Urinary tract infections (UTIs) with Escherichia coli
- Strep throat (group A Streptococcus)
- Sepsis, especially in infants and elderly adults
3. Other Infectious Agents
Fungal and parasitic infections, though less common, can also produce a high fever:
- Histoplasmosis
- Malaria
- Tuberculosis
4. Inflammatory and Autoimmune Conditions
Systemic inflammation can elevate temperature. Examples include:
- Rheumatoid arthritis flare-ups
- Systemic lupus erythematosus (SLE)
- Adult-onset Still’s disease
5. Medication and Vaccine Reactions
Some drugs and vaccines can raise body temperature as a side effect. Common triggers are:
- Antibiotics (e.g., amoxicillin, ceftriaxone)
- Vaccinations (e.g., MMR, influenza vaccine)
- Immunomodulators such as TNF‑α inhibitors
6. Heat‑Related Illnesses
Excessive heat exposure can lead to heat exhaustion or heat stroke, both of which present with a high core temperature.
