Fever: what it usually means
Also searched as: high temperature, chills and fever, running a temperature · Updated 19 Aug 2026 · Educational — not medical advice
The short answer: A fever means your body temperature is higher than normal, usually because your immune system is fighting an infection. Most fevers in adults are caused by common viral illnesses and settle within a few days with rest and fluids. A fever itself is not dangerous for most healthy adults, but very high temperatures, fevers that last many days, or ones with severe symptoms alongside them need prompt medical assessment to rule out more serious infections.
When to get medical help
A baby under 3 months with a temperature of 38°C (100.4°F) or higher — go to an emergency department now, unless a doctor can see your baby within 2 hours. An adult with a fever above 103°F (39.4°C) needs prompt medical care.
Fever lasting more than three days without improvement should be evaluated by a doctor.
Fever with a stiff neck, severe headache, rash, or confusion needs emergency attention. Go to an emergency department now.
Fever with difficulty breathing, a seizure, being hard to wake, or blue lips, tongue or nails — call emergency services now.
Fever with vomiting that won't stop or severe abdominal pain — see a doctor today.
Fever in someone with a chronic illness or weakened immune system should be checked sooner rather than later.
Common causes
- Viral infections — Common colds, flu, and other viral illnesses are the most frequent cause of fever in adults and children.
- Bacterial infections — Infections such as urinary tract or throat infections can trigger fever and often need specific treatment.
- Inflammatory conditions — Certain autoimmune or inflammatory conditions can cause recurring or persistent low-grade fevers.
- Heat exposure — Overheating from hot weather or intense exercise can temporarily raise body temperature.
- Post-vaccination response — A mild, short-lived fever is a normal immune response after some vaccinations.
What helps
- Rest and drink plenty of fluids to prevent dehydration.
- Dress lightly and keep the room at a comfortable temperature rather than overheating.
- For adults, a lukewarm (not cold) bath or sponge may help comfort; do not sponge or undress children to cool them.
- Monitor your temperature and watch for new or worsening symptoms.
Which doctor to consult
Start with your family doctor for evaluation and testing; for children, a pediatrician is the right first stop.
Read more about the causes
Questions people ask
Is it okay to let a mild fever run its course?
Yes, in healthy adults a mild fever often helps the body fight infection and can be allowed to run its course while you rest and stay hydrated. Fever-reducing medicine can help with comfort but isn't always necessary for mild fevers.
What temperature counts as a high fever?
In adults, a temperature above 103°F (39.4°C) is generally considered high and worth medical attention, especially if it doesn't respond to usual measures. Any fever in a very young infant should always be checked by a doctor promptly.
Related symptoms
Sources
- High temperature (fever) in adults — NHSnhs.uk
- Fever — MedlinePlus Medical Encyclopediamedlineplus.gov
- Fever: What Is Considered a Fever? — Cleveland Clinicmy.clevelandclinic.org
- Fever in under 5s: assessment and initial management — NICEnice.org.uk
This guide is educational and doesn't replace a clinical assessment. If a symptom is severe, rapidly worsening, or worrying you, see a doctor — being checked and reassured is never a wasted visit.