A fever means your body is raising its internal temperature, almost always as a deliberate defense against an infection or illness. An oral temperature of 100°F (37.8°C) or higher is generally considered a fever. Rather than being the illness itself, a fever is your immune system’s way of fighting back, creating an environment that’s harder for viruses and bacteria to survive in while boosting the performance of your own immune cells.
How Your Body Creates a Fever
Your brain has a built-in thermostat located in a small region called the hypothalamus. Normally, it keeps your core temperature hovering around 98.6°F. When your immune system detects a threat, like a virus or bacterial infection, it releases signaling molecules called cytokines. These molecules travel to the brain and trigger the release of a chemical called prostaglandin E2, which essentially turns up the thermostat’s set point.
Once the set point rises, your body treats its current normal temperature as “too cold” and starts working to heat up. Blood vessels near your skin constrict to hold in warmth, which is why you might look pale or feel cold even though your temperature is climbing. If that’s not enough to reach the new set point, your muscles start contracting involuntarily. That’s shivering, and it generates heat through rapid movement. This is why you can feel freezing and pile on blankets at the start of a fever, even though your body is actually getting warmer.
Why Fever Actually Helps You
Fever isn’t just a symptom to endure. It’s a functional part of your immune response. Research from the National Institutes of Health found that when T cells (a critical type of immune cell) were exposed to fever-range temperatures around 102.2°F, they multiplied faster, had higher metabolic activity, and released more of the signaling molecules that coordinate your body’s immune attack. Every major category of T cell tested performed better at fever temperature than at normal body temperature.
At the same time, many pathogens reproduce best at normal human body temperature. Raising the temperature a few degrees can slow their ability to replicate, giving your immune system a head start. This is why mild to moderate fevers are generally considered helpful rather than harmful, and why treating a fever isn’t always necessary if you’re otherwise feeling okay.
What Causes a Fever
Infections are the most common trigger. Colds, flu, COVID-19, urinary tract infections, ear infections, strep throat, and stomach bugs all routinely cause fevers. But infections aren’t the only explanation. Autoimmune conditions like lupus and rheumatoid arthritis can trigger fevers because the immune system is attacking the body’s own tissues and producing the same inflammatory signals. Certain medications can cause what’s known as a drug fever, where the body reacts to the medication itself. Heat exhaustion and heatstroke raise body temperature through a completely different mechanism: the body simply can’t cool itself fast enough. Some cancers, particularly lymphomas, also cause recurring fevers.
If you have a fever with no obvious cold or flu symptoms, it doesn’t necessarily mean something serious is wrong. But a fever lasting more than a few days without a clear cause is worth investigating.
How to Read Your Thermometer
Where you take your temperature matters, because different parts of the body give slightly different readings. The standard reference point is an oral (mouth) reading, where 98.6°F is considered the average normal.
- Rectal: Reads 0.5°F to 1°F higher than oral. This is the most accurate method for infants.
- Ear (tympanic): Reads 0.5°F to 1°F higher than oral.
- Armpit (axillary): Reads 0.5°F to 1°F lower than oral.
- Forehead (temporal): Reads 0.5°F to 1°F lower than oral.
So an armpit reading of 99°F and a rectal reading of 99°F represent very different actual temperatures. If you’re using an armpit or forehead thermometer and get a reading that seems borderline, keep this offset in mind. There’s no exact universal conversion between methods, but these ranges give you a reliable estimate.
Fever Ranges and What They Mean
Not all fevers carry the same significance. A low-grade fever in the 100°F to 102°F range is common with routine viral infections and typically resolves on its own within a few days. It means your immune system is responding normally.
A temperature of 103°F (39.4°C) or higher in an adult warrants a call to your healthcare provider. At this level, you’re more likely to feel significant discomfort, including headaches, muscle aches, and fatigue, and the fever may signal a more aggressive infection that needs attention.
Temperatures above 106.7°F (41.5°C) constitute a medical emergency called hyperpyrexia. At this extreme, the heat itself can damage organs and brain tissue. This is rare with typical infections and more commonly associated with heatstroke, certain drug reactions, or severe sepsis. It requires immediate emergency treatment.
Fever Thresholds for Children
Children, especially infants, have different thresholds that parents should know. For babies under 3 months old, a rectal temperature of 100.4°F (38°C) or higher needs prompt medical evaluation. At this age, even a low fever can indicate a serious bacterial infection because the immune system is still immature. Infants under 28 days with any fever require immediate hospital evaluation regardless of how well they appear.
For babies between 3 and 6 months, a rectal temperature above 102°F (38.9°C) is the key threshold, or any fever paired with unusual irritability, sluggishness, or discomfort. Between 7 and 24 months, the same 102°F threshold applies if the fever persists for more than a day. For children under 3 years, a temperature above 102.2°F (39°C) with no obvious source carries a higher risk of a hidden bacterial infection, particularly if the child hasn’t completed their routine vaccinations.
Bringing a Fever Down
You don’t always need to treat a fever. If you’re an adult with a low-grade fever and tolerable symptoms, letting it run its course allows your immune system to do its job. But if you’re miserable, not sleeping, or the fever is climbing toward 103°F, reducing it makes sense for comfort and safety.
The two main over-the-counter options are acetaminophen and ibuprofen. Both work, but they’re not identical. A meta-analysis comparing the two in children found that ibuprofen lowered temperature slightly more than acetaminophen and was more likely to bring the fever down to normal. Within four hours, about 1 in 8 additional children became fever-free with ibuprofen compared to acetaminophen. Over a 4-to-24-hour window, that improved to about 1 in 6. Ibuprofen also reduces inflammation, which can help with body aches that accompany many fevers. Acetaminophen is gentler on the stomach and remains the safer choice for very young infants and people who can’t take anti-inflammatory drugs.
Beyond medication, staying hydrated is one of the most important things you can do. Fever increases fluid loss through the skin, and the higher your temperature, the more water your body burns through. For every degree Celsius above 100.4°F, your body needs roughly 10% more fluid replacement than normal. That adds up quickly during a multi-day fever. Water, diluted juice, broth, and oral rehydration solutions all work. If you notice dark urine, dry mouth, or dizziness, you’re falling behind on fluids.
Light clothing, a comfortable room temperature, and rest round out the basics. Ice baths and alcohol rubs are outdated approaches that can cause shivering, which actually raises your core temperature further.
Warning Signs That Need Attention
Most fevers resolve within a few days and don’t cause lasting harm. But certain symptoms alongside a fever signal something more serious. A rash of small purple or red spots that don’t fade when you press on them (petechiae) can indicate a blood infection. Difficulty breathing, persistent vomiting, confusion, a stiff neck, or a severe headache all warrant urgent evaluation. In children, inconsolable crying or extreme lethargy are red flags regardless of the temperature reading.
For adults, any fever of 103°F or higher, a fever lasting more than three days, or a fever that returns after seeming to resolve deserves medical follow-up. For children, the thresholds are lower and the timeline shorter, especially in the first year of life.