A fever starts at 100.4°F (38°C) for most measurement methods, and the single most important thing you can do is keep the person hydrated, comfortable, and resting. Most fevers are the body’s natural response to infection and resolve on their own within a few days. What matters is knowing how to ease discomfort, when to use medication, and which warning signs mean it’s time to get medical help.
How to Tell It’s Actually a Fever
Average body temperature is 98.6°F (37°C), but normal ranges anywhere from 97°F to 99°F depending on the person, time of day, and where you measure. A true fever is generally defined as a rectal, ear, or oral temperature of 100.4°F (38°C) or higher. An armpit reading of 99°F (37.2°C) or higher also qualifies, though armpit readings tend to be less precise.
Adults with fevers of 103°F (39.4°C) or higher will typically look and act noticeably sick. But even lower fevers can cause fatigue, chills, sweating, and muscle aches. If you’re unsure whether someone feels warm because of a fever or just because they’ve been active or bundled up, wait 15 minutes in a cool room and check again with a thermometer.
The Three Basics: Fluids, Rest, and Cooling
Fever increases fluid loss through sweating and faster breathing. Dehydration is the most common complication, so steady fluid intake is the top priority. Water, diluted juice, and broth all work. For babies under one year, use an oral rehydration solution like Pedialyte, which replaces both water and electrolytes in the right proportions. Pedialyte ice pops can help if a child resists drinking.
Rest genuinely matters here. Physical activity raises body temperature on its own, which works against recovery. The person doesn’t need to stay in bed all day, but this isn’t the time for exercise, errands, or pushing through a workday.
Keep the room cool and dress in light clothing. A single sheet or light blanket is enough for sleeping. The instinct to pile on blankets when someone has chills is understandable, but heavy layers trap heat and can push the temperature higher. If the person is shivering, one light layer is fine until the shivering passes.
When and How to Use Fever-Reducing Medication
Fever-reducing medication isn’t always necessary. A fever under 102°F in an otherwise healthy adult or older child often doesn’t need medication at all, especially if the person isn’t miserable. The goal of medication is comfort, not hitting a specific number on the thermometer.
When discomfort is significant, acetaminophen (Tylenol) and ibuprofen (Advil, Motrin) are the standard options. Acetaminophen can be given every 4 to 6 hours, with no more than 5 doses in 24 hours. Ibuprofen is given every 6 to 8 hours, with no more than 4 doses in 24 hours. For children, doses are based on weight, not age, so check the packaging carefully or call your pharmacist if you’re unsure.
One critical rule: never give aspirin to children or teenagers. Aspirin use during viral illnesses like the flu or chickenpox has been linked to Reye’s syndrome, a rare but life-threatening condition that causes swelling in the liver and brain. Without proper treatment, Reye’s syndrome can be fatal within days. This applies to any product containing aspirin, so read labels. The only exception is children on long-term aspirin therapy for specific chronic conditions like Kawasaki disease, prescribed by their doctor.
Skip the Cold Bath
You might assume a cold or lukewarm bath would help bring a fever down, but research shows it’s not worth it. When cool water hits the skin of a feverish person, the body fights back with shivering, blood vessel constriction, and increased internal heat production. The net result is only about a 0.3°C drop in temperature on average, even when combined with fever-reducing medication. Meanwhile, the experience is uncomfortable and can cause significant distress in children. A cool washcloth on the forehead is harmless and may feel soothing, but don’t count on baths as an effective cooling strategy.
Watch for Dehydration
Because fever accelerates fluid loss, monitoring for dehydration is essential, especially in young children who can’t always tell you they’re thirsty. In infants and toddlers, the signs to watch for include no wet diapers for three hours or longer, a dry mouth, no tears when crying, sunken eyes or cheeks, and a sunken soft spot on top of the skull. You can also gently pinch the skin on the back of the hand or forearm. If it doesn’t flatten back right away, that suggests dehydration.
In older children and adults, key signs are dark or infrequent urination, dry lips and mouth, dizziness when standing, and unusual fatigue beyond what the fever itself would cause. Small, frequent sips are easier to tolerate than large glasses, especially if nausea is present.
Fevers in Babies Need Immediate Attention
The rules are different for very young infants. Any baby under 3 months old with a rectal temperature of 100.4°F or higher needs medical evaluation right away, even if the baby seems fine otherwise. Young infants can deteriorate quickly, and fever at this age can signal serious infections that need prompt treatment. Don’t give fever-reducing medication to a baby this young without guidance from a healthcare provider, and don’t wait to see if the fever resolves on its own.
The American Academy of Pediatrics focuses particular attention on infants between 8 and 60 days old with fevers at or above 100.4°F. This age group often requires specific testing to rule out bacterial infections, even when the baby appears well.
How Long a Fever Typically Lasts
Most fevers are caused by viral infections and follow a predictable pattern. Symptoms typically peak around 3 days after onset and then gradually improve. A fever that lasts 2 to 3 days with a clear cause (a cold, the flu, a stomach bug) is usually nothing to worry about as long as the person is staying hydrated and the temperature isn’t climbing above 103°F.
Distinguishing a viral fever from a bacterial one based on symptoms alone is difficult. Even doctors find it challenging. Routine blood markers like white blood cell count and C-reactive protein can offer clues, but they vary too much from person to person to be definitive. In practice, a fever that persists beyond 3 to 4 days without improving, or one that initially gets better and then spikes again, is more likely to involve a bacterial component that may need treatment.
Red Flags That Need Emergency Care
Most fevers are uncomfortable but harmless. However, certain symptoms alongside a fever point to potentially dangerous conditions like meningitis, sepsis, or organ involvement. Seek immediate medical attention if a fever occurs with any of the following:
- Stiff neck with pain when bending the head forward, or unusual sensitivity to bright light
- Mental confusion, strange behavior, or altered speech
- Severe headache that doesn’t respond to medication
- New rash, particularly one that doesn’t fade when you press on it
- Difficulty breathing or chest pain
- Persistent vomiting that prevents keeping fluids down
- Seizures or convulsions
- Abdominal pain or pain when urinating
For adults, a fever of 103°F or higher that doesn’t come down with medication also warrants medical evaluation. And any fever in a person with a weakened immune system, whether from chemotherapy, an organ transplant, or a chronic illness, should be taken seriously regardless of the number on the thermometer.