Fever

What is it?

Fever is a regulated rise in body temperature set by the hypothalamus in response to pyrogens — substances released by immune cells during infection or inflammation. It is a normal, often beneficial immune response that enhances pathogen clearance. Most fevers in healthy adults are caused by self-limiting viral infections. However, very high fevers (above 40°C/104°F), fever in immunocompromised patients, or fever with specific associated symptoms require prompt evaluation. Fever of unknown origin (FUO) — persistent fever without an identified cause — warrants comprehensive workup.

Common causes

When to see a doctor

• 1Temperature above 40°C (104°F) in an adult
• 2Fever with severe headache and stiff neck (possible meningitis)
• 3Fever with confusion, slurred speech, or altered consciousness
• 4Fever in an immunocompromised person (cancer, HIV, transplant recipient)
• 5Fever that lasts more than 3 days without improvement
• 6Fever with a new rash, especially one that is spreading or non-blanching

What you can do

• ✓Stay well hydrated — fever increases fluid loss through sweating
• ✓Rest; avoid strenuous activity while febrile
• ✓Dress lightly; avoid over-bundling which traps heat
• ✓Acetaminophen or ibuprofen reduce fever and relieve discomfort
• ✓Lukewarm (not cold) sponging can help if fever is very high and discomfort is significant
• ✓Do not give aspirin to children — risk of Reye syndrome

Conditions that cause this symptom

Medications that may help

Treatment depends on the underlying cause. Always consult your provider before starting any medication.

Frequently asked questions