Fever begins at a body temperature of 100.4°F (38°C) when measured rectally or at the ear, or 100°F (37.8°C) when measured orally. The symptoms that come with it, from chills and muscle aches to sweating and fatigue, are actually signs that your body is actively fighting an infection. Here’s what to expect and what those symptoms mean.
Common Symptoms of Fever
A fever rarely shows up alone. Most people experience a combination of these symptoms:
- Chills and shivering: Your body generates heat by making your muscles contract rapidly. This is why you feel cold even though your temperature is rising.
- Sweating: Once your temperature peaks and starts to drop, your body releases heat through sweat.
- Headache
- Muscle aches
- Loss of appetite
- General weakness and fatigue
- Dehydration
- Irritability (especially in young children)
The chills and shivering phase often confuses people because it feels like you’re freezing, not overheating. When you wrap up in a blanket because you feel chilled, you’re actually helping your body do what it’s already trying to do: raise your internal temperature. The shivering stops once your body reaches the new, higher temperature it’s aiming for.
Why Your Body Creates These Symptoms
Fever isn’t a malfunction. It’s a deliberate response orchestrated by a region deep in your brain that acts as your body’s thermostat. When you have an infection, your immune system sends chemical signals to this thermostat, which then raises the target temperature. Think of it like turning the dial on your home thermostat from 68°F to 102°F. Your body “feels” cold relative to the new, higher target, so it kicks into gear to produce more heat.
To raise your temperature, your brain triggers several things at once: your muscles shiver to generate heat, blood vessels near your skin constrict to prevent heat loss (which is why your skin can look pale), and your heart rate increases to circulate warmer blood faster. This coordinated response explains why a fever feels so physically draining. Your body is burning extra energy on top of fighting whatever infection triggered the response in the first place.
How Fever Feels Different by Temperature
Not all fevers feel the same. A low-grade fever, generally between 100.4°F and 102°F, often produces mild achiness, slight chills, and a general feeling of being “off.” You might not even realize you have a fever without a thermometer. Many people push through a low-grade fever at work or school without much trouble.
Higher fevers, above 103°F in adults, tend to bring more intense symptoms: heavy shivering, significant muscle pain, pronounced weakness, and sometimes confusion or unusual drowsiness. At these temperatures, dehydration becomes a bigger concern because your body loses fluids faster through sweating and increased breathing rate.
Fever Symptoms in Children
Children often spike higher fevers than adults do, and their symptoms can look different. Irritability, clinginess, flushed cheeks, and refusal to eat or drink are common signs in kids who can’t yet describe how they feel. Very young infants may become unusually quiet or limp rather than fussy.
For babies under 3 months, a rectal thermometer is the most reliable way to check. Any reading of 100.4°F or higher in this age group warrants prompt medical attention, even if the baby otherwise looks fine. For children between 3 and 6 months, rectal and armpit thermometers are both reasonable options, though armpit readings tend to run slightly lower than the true core temperature.
Febrile Seizures
One symptom unique to young children is febrile seizures, which occur in kids between 6 months and 5 years old, with the highest risk between ages 1 and 3. These seizures can involve loss of consciousness, uncontrollable shaking or stiffening of the arms and legs, eye rolling, and loss of bodily control such as drooling or vomiting. They’re frightening to witness but are typically brief. The most common type lasts a few seconds to 15 minutes and happens only once within a 24-hour period. Complex febrile seizures last longer than 15 minutes or occur more than once in a day.
Taking an Accurate Temperature
Where you place the thermometer matters. Rectal readings are the most accurate but also the most invasive, which is why they’re mainly recommended for infants. Oral readings work well for older children and adults. Armpit temperatures are the least accurate and tend to read about a degree lower than oral measurements. Forehead (temporal artery) thermometers offer a good balance of accuracy and convenience for all ages.
One important detail: there’s no reliable formula for converting between measurement sites. You can’t simply add a degree to an armpit reading and call it equivalent to an oral temperature. The best approach is to use the same method consistently so you can track changes over time, and tell your doctor which method you used when reporting a temperature.
When a Fever Needs Treatment
Fever itself is a defense mechanism, not a disease. A moderate fever in an otherwise healthy person actually helps the immune system work more efficiently. For this reason, many pediatricians and doctors recommend treating the discomfort of a fever rather than the number on the thermometer. If you or your child feel reasonably comfortable, there’s no strict need to bring the temperature down.
That said, treating symptoms makes sense when a fever is causing significant misery: pounding headache, severe body aches, or difficulty sleeping. Staying hydrated is the single most important thing during any fever, since your body loses fluids faster than normal. Light clothing and a comfortable room temperature help more than piling on blankets, especially once the chills phase has passed.
For infants 8 to 60 days old with a temperature at or above 100.4°F, the American Academy of Pediatrics has specific evaluation guidelines that include blood work and urine tests, because young babies can have serious infections without showing obvious signs beyond the fever itself.
Symptoms That Signal Something Serious
Most fevers resolve on their own within a few days. But certain symptoms alongside a fever point to something that needs immediate attention: a stiff neck combined with sensitivity to light, a rash of small purple or red spots that don’t fade when you press on them, persistent vomiting that prevents you from keeping fluids down, difficulty breathing, confusion or unusual drowsiness, or a fever above 103°F that doesn’t respond to standard treatment.
In infants under 3 months, any fever at all is worth a call to your pediatrician. In children between 3 months and 3 years, a fever lasting more than three days or climbing above 102°F deserves medical evaluation, especially if the child seems unusually lethargic or isn’t producing wet diapers at their normal rate.