Fever
Overview
Fever is an elevation of body temperature above the normal range, typically defined as 100.4°F (38°C) or higher when measured orally. It represents the body's natural defense mechanism against infection and illness. When the immune system detects harmful invaders like viruses or bacteria, it triggers the hypothalamus—the brain's temperature control center—to raise body temperature. This creates a less favorable environment for pathogens and enhances immune function. Fever itself is not an illness but rather a symptom indicating that the body is fighting something. While often concerning, especially in children, most fevers are caused by common infections and resolve without specific treatment. The degree of fever doesn't always correlate with the severity of illness; a high fever can accompany a minor viral infection, while some serious conditions may produce only mild temperature elevation. Fever may be accompanied by chills, sweating, headache, muscle aches, weakness, and loss of appetite.
Common causes
- Viral infections such as influenza, common cold, COVID-19, or gastroenteritis
- Bacterial infections including strep throat, urinary tract infections, pneumonia, or skin infections
- Childhood immunizations (usually mild, short-lived fever)
- Ear infections, particularly in young children
- Sinus infections
- Gastroenteritis (stomach flu)
- Upper respiratory tract infections
- Teething in infants (though typically causes only low-grade fever if any)
Less common causes
- Autoimmune diseases such as rheumatoid arthritis or lupus
- Inflammatory bowel disease
- Blood clots or deep vein thrombosis
- Certain cancers, particularly lymphoma or leukemia
- Medication reactions or drug fever
- Heat exhaustion or heat stroke
- Tuberculosis
- Appendicitis
- Meningitis or encephalitis
- Endocarditis (infection of heart valves)
- Malaria or other tropical diseases in travelers
- Thyroid disorders
Self-care
- Stay well-hydrated by drinking plenty of water, clear broths, or electrolyte solutions
- Get adequate rest to help your body fight the underlying cause
- Dress in lightweight, breathable clothing and keep room temperature comfortable
- Take over-the-counter fever reducers like acetaminophen or ibuprofen as directed on the package (never give aspirin to children due to risk of Reye's syndrome)
- Use lukewarm sponge baths if needed for comfort; avoid cold baths which can cause shivering and raise core temperature
- Monitor temperature regularly and keep a record of readings and associated symptoms
- Avoid bundling up excessively, which can trap heat and raise temperature further
- Eat light, easily digestible foods if you have an appetite
When to seek care
Seek immediate emergency care if fever is accompanied by severe headache with stiff neck, difficulty breathing, chest pain, persistent vomiting, confusion or altered mental state, seizures, severe abdominal pain, rash that doesn't fade when pressed, or signs of dehydration. For adults, contact a healthcare provider if fever reaches 103°F (39.4°C) or higher, persists more than three days, occurs after recent travel to tropical areas, or is accompanied by unexplained symptoms. For infants under 3 months, any fever of 100.4°F (38°C) or higher requires immediate medical attention. For children aged 3-36 months, seek care for fever above 102.2°F (39°C) or any fever lasting more than 24 hours. Contact your doctor if you have a fever and are immunocompromised, undergoing cancer treatment, or have chronic medical conditions. Trust your instincts—if you or your child appears seriously ill regardless of temperature reading, seek medical evaluation.
Related specialists
- Primary care physician or family medicine doctor
- Pediatrician for children
- Infectious disease specialist for persistent or unexplained fevers
- Emergency medicine physician for acute severe symptoms
- Internist for adult complex cases
Frequently asked questions
A fever is generally defined as a body temperature of 100.4°F (38°C) or higher when measured orally. Rectal temperatures tend to run about one degree higher, while armpit (axillary) temperatures run about one degree lower. Normal body temperature varies throughout the day and among individuals, typically ranging from 97°F to 99°F (36.1°C to 37.2°C). The method of measurement matters, so it's important to note how temperature was taken when reporting to healthcare providers.
This depends on your comfort level and overall condition. Fever is a natural immune response that may help fight infection, so low-grade fevers don't necessarily need treatment if you're otherwise comfortable. However, reducing fever with medication can help you feel better, rest more easily, and stay hydrated. Treat fever if it's causing significant discomfort, preventing sleep, or contributing to dehydration. Always treat fever in young children who seem uncomfortable. The goal is comfort rather than achieving a completely normal temperature. Consult your healthcare provider about the best approach for your specific situation.
Yes, absolutely. Not all infections cause fever, and some serious infections may not produce significant temperature elevation. Older adults, people with weakened immune systems, and those taking certain medications may not develop fever even when seriously ill. Additionally, some infections characteristically produce little or no fever. The absence of fever doesn't rule out infection or other illness. Pay attention to other symptoms like unusual fatigue, pain, changes in breathing, confusion, or just feeling significantly unwell, and seek medical attention when appropriate regardless of temperature.
For adults, a fever lasting more than three days warrants medical evaluation, especially if the cause isn't clear or symptoms worsen. In children, the timeline is shorter—contact your pediatrician if fever persists beyond 24 hours in children under 2 years, or beyond 72 hours in older children. However, these are general guidelines. If you have other concerning symptoms, are getting worse rather than better, or the fever keeps returning after initially improving, don't wait—contact your healthcare provider sooner. Some viral infections can cause fever on and off for a week or more, but this should be evaluated by a medical professional.
Fever patterns can sometimes provide clues to underlying causes, though they're not definitive for diagnosis. Continuous fever that stays elevated throughout the day often accompanies pneumonia or typhoid. Intermittent fever that comes and goes may suggest abscesses or certain infections. Relapsing fever that resolves then returns days later might indicate malaria or other specific conditions. However, many illnesses don't follow predictable patterns, and patterns can overlap between different conditions. Rather than trying to interpret fever patterns yourself, keep a detailed record of temperature readings and associated symptoms to share with your healthcare provider, who can interpret them in the context of your complete clinical picture.