A fever starts at a body temperature of 100.4°F (38°C) or higher, and it typically brings a recognizable set of symptoms: chills, body aches, sweating, and a general feeling of being unwell. Most fevers from common infections last three to four days and resolve on their own, but the specific symptoms you experience depend on how high the temperature climbs and what’s causing it.
How a Fever Works in Your Body
Your brain has a built-in thermostat that normally keeps your body temperature hovering around 98.6°F. When your immune system detects an infection, immune cells release signaling molecules that travel to this thermostat and essentially turn the dial up. Your brain then treats your normal body temperature as “too cold” and triggers a cascade of responses to generate heat: your muscles contract rapidly (causing chills and shivering), blood vessels near your skin tighten to trap warmth inside, and your metabolic rate increases.
This is why you feel freezing cold at the start of a fever even though your temperature is actually rising. Once your body reaches the new, higher set point, the chills stop. Later, when the fever breaks, your brain resets the thermostat back to normal, and you suddenly feel too hot. That’s when the sweating kicks in as your body works to cool itself down.
The Most Common Symptoms
Fever rarely shows up alone. The process of raising your body temperature produces its own set of symptoms, and the underlying infection often adds more on top. The most recognizable signs include:
- Chills and shivering, caused by rapid muscle contraction as your body generates heat. These often appear before a thermometer even registers a higher temperature.
- Sweating, especially as the fever breaks or fluctuates.
- Muscle aches and body soreness, partly from the shivering itself and partly from the inflammatory molecules your immune system releases.
- Headache, which tends to worsen as fever climbs higher.
- General fatigue and weakness, since your body is diverting energy toward fighting infection and raising its temperature.
- Loss of appetite, which is common across nearly all fever-causing illnesses.
- Warm or flushed skin, particularly noticeable on the forehead and cheeks once the temperature has reached its peak.
Not everyone experiences all of these at once. A low-grade fever (just above 100.4°F) might feel like mild fatigue and slight warmth, while a fever above 103°F often brings intense chills, significant body aches, and difficulty concentrating.
Dehydration: The Hidden Companion
Fever increases your body’s water loss through sweating and faster breathing, and the higher the fever, the worse the dehydration can get. In adults, signs of dehydration during a fever include extreme thirst, dark-colored urine, urinating less often than usual, dizziness, and tiredness that goes beyond what the fever itself would cause. Your skin may also lose its elasticity: if you pinch the back of your hand and the skin doesn’t flatten back quickly, that’s a sign you need more fluids.
In young children, dehydration shows up differently. Watch for fewer wet diapers (none in three hours is a warning sign), a dry mouth, no tears when crying, sunken eyes, and unusual crankiness or low energy. Dehydration during a fever can become serious faster in small children than in adults.
Fever Symptoms in Babies and Young Children
Infants and toddlers can’t tell you what they’re feeling, so fever symptoms show up as behavioral changes. Babies with a fever are typically irritable and may refuse to eat or sleep well. The higher the fever climbs, the more disinterested and fussy children tend to become. In rare cases, a very high fever can make a child unusually drowsy, limp, or unresponsive, which is a sign that something more serious may be happening.
Children between 6 months and 5 years old are also at risk for febrile seizures, which are seizures triggered by a rapid rise in body temperature. These look alarming: the child may lose consciousness, shake or jerk uncontrollably in the arms and legs, roll their eyes, or lose bladder control. Most febrile seizures last less than a few minutes and don’t cause lasting harm, but they’re most common between ages 1 and 3 and tend to happen as the fever is spiking rather than after it’s been high for a while.
How Thermometer Readings Compare
The number on your thermometer depends on where you take the reading, which can create confusion about whether you actually have a fever. Rectal and ear thermometers read about 0.5 to 1°F higher than an oral thermometer. Armpit and forehead readings run about 0.5 to 1°F lower than oral. So a forehead reading of 99.5°F and an oral reading of 100.4°F could represent the same actual body temperature.
For infants, rectal thermometers give the most accurate reading. For older children and adults, oral or ear thermometers are reliable enough for home use. The key threshold stays the same regardless of method: a rectal, ear, or oral temperature of 100.4°F (38°C) or higher counts as a fever, while an armpit reading of 99°F (37.2°C) or higher does.
How Long Fever Symptoms Typically Last
Fevers from common viral infections like colds, the flu, or stomach bugs generally last three to four days. During that window, your symptoms may fluctuate: the fever often rises in the late afternoon and evening, then dips in the morning, giving a false sense of recovery before climbing again. The chills, aches, and fatigue tend to follow the same pattern, worsening when the fever spikes and easing when it drops.
A fever that persists beyond three days, or one that goes away and then returns after a day or two, is worth getting checked by a healthcare provider. This pattern can suggest a bacterial infection that needs treatment, or an infection that your immune system is struggling to clear on its own.
Symptoms That Signal an Emergency
Most fevers are uncomfortable but not dangerous. However, certain symptoms alongside a fever point to a potentially serious condition. A fever above 104°F (40°C) in an adult warrants a call to your doctor. Beyond that, the following combinations require immediate medical attention:
- Stiff neck with fever, which can indicate meningitis
- Confusion or difficulty staying alert
- Trouble breathing or chest tightness
- Seizures or loss of consciousness
- Severe pain anywhere in the body
- Swelling or visible inflammation
- Pain when urinating or foul-smelling urine, suggesting a urinary tract infection that may be spreading
For infants under 3 months old, any fever of 100.4°F or higher is considered an emergency regardless of other symptoms, because their immune systems are too immature to reliably fight off serious infections.