A fever means your body has deliberately raised its internal temperature, almost always as a defense against something it perceives as a threat. The most common reason is a viral or bacterial infection, but medications, autoimmune conditions, and other triggers can also be responsible. Normal body temperature is actually lower than the long-cited 98.6°F. A large study analyzing over 677,000 temperature measurements spanning 157 years found that average human body temperature has dropped roughly 1°F since the 1800s, meaning your personal baseline may sit closer to 97.5°F or 98°F.
What Counts as a Fever
Harvard Health defines fever ranges for adults as follows:
- Low-grade: 99.1 to 100.4°F (37.3 to 38.0°C)
- Moderate: 100.6 to 102.2°F (38.1 to 39.0°C)
- High-grade: 102.4 to 105.8°F (39.1 to 41.0°C)
Where you measure matters. Rectal readings are the most accurate, followed closely by oral thermometers. Ear thermometers can be thrown off by earwax or ear infections, and forehead thermometers lose accuracy in direct sunlight, cold air, or if the forehead is sweaty. For consistency, compare readings taken from the same spot each time rather than trying to convert between locations.
How Your Body Creates a Fever
Your brain has a built-in thermostat in a region called the hypothalamus, and fever happens when that thermostat gets reset to a higher temperature. The process starts when your immune cells detect a threat, whether that’s a virus, bacteria, or another trigger. Those immune cells release signaling molecules (primarily small proteins called cytokines) that travel to the brain through the bloodstream and through nerve pathways, including the vagus nerve.
Once those signals reach the hypothalamus, they trigger the production of a chemical messenger called prostaglandin E2. This is the molecule that actually shifts the set point upward. Your hypothalamus then treats your normal body temperature as “too cold” and activates warming mechanisms: shivering, constricting blood vessels near the skin, and driving you to curl up under blankets. This is why you feel chills at the start of a fever even though your temperature is rising. Common fever-reducing medications work by blocking the enzyme that produces prostaglandin E2, which is why they can bring the set point back down.
Infections: The Most Common Cause
The vast majority of fevers are caused by infections. Viral infections, including colds, flu, COVID-19, and stomach bugs, tend to produce widespread symptoms: body aches, fatigue, runny nose, cough, sore throat, and a low-grade fever. These whole-body symptoms reflect your immune system mounting a broad response. Most viral illnesses last about a week, sometimes stretching to two weeks, and resolve on their own.
Bacterial infections tend to cause more localized problems. A severe sore throat concentrated on one side, a red and tender patch of skin, or ear pain with pressure are more characteristic of bacteria. Bacterial infections are also more likely to need treatment with antibiotics, while viral infections generally don’t respond to them.
One pattern worth knowing: a viral illness that starts improving and then suddenly gets worse, with a higher fever and new pain, can signal a secondary bacterial infection that moved in while your immune system was occupied. Similarly, symptoms that persist beyond 10 days without improvement, or that improve and then worsen, are a reason to get evaluated.
Non-Infectious Causes
Not every fever means you’re fighting an infection. Autoimmune conditions like lupus and rheumatoid arthritis can trigger fevers because the immune system is attacking the body’s own tissues, generating the same inflammatory signals that normally respond to germs. These fevers tend to be recurrent and often accompany flares of other symptoms like joint pain, rashes, or fatigue.
Certain medications and substances can also raise body temperature through different mechanisms. Stimulants like amphetamines, cocaine, and MDMA increase heat production directly. Some antidepressants, when combined with each other or with other serotonin-boosting drugs, can cause a dangerous reaction called serotonin syndrome that includes high fever, agitation, and muscle rigidity. Antihistamines and some older antidepressants can interfere with the body’s ability to cool itself by blocking sweating. Even withdrawal from alcohol or sedatives can trigger fevers. If you develop a fever after starting a new medication or combining substances, that timing is important information for your doctor.
Why Fever Actually Helps You
Fever feels miserable, but it exists for a reason. Elevated body temperature directly hampers many pathogens by pushing them outside the temperature range where they replicate best. Historically, induced fever was even used as a treatment for syphilis before antibiotics existed, and heat therapy showed effectiveness against poliovirus. More recently, researchers have found that a common human coronavirus replicated less effectively at higher temperatures, and that the body’s antiviral signaling pathways work more efficiently at fever-range temperatures.
Beyond slowing down invaders, fever enhances multiple parts of the immune response. It improves the movement of white blood cells to infection sites, strengthens the ability of immune cells to present pieces of pathogens to other immune cells (which speeds up targeted responses), boosts antibody production, and influences how certain immune cells develop. This is why many doctors now recommend letting mild to moderate fevers run their course rather than immediately suppressing them, particularly in otherwise healthy adults.
Fever in Infants and Children
The rules change significantly for babies. Any fever in an infant younger than 3 months old requires a call to your pediatrician, regardless of how high the temperature is or how well the baby seems. Young infants have immature immune systems, and fever can be the only visible sign of a serious infection that needs rapid evaluation. For older children, the same general principles apply as adults: the fever itself is a defense mechanism, and how the child looks and acts matters more than the exact number on the thermometer.
Red Flags That Need Immediate Attention
Most fevers are uncomfortable but not dangerous. However, certain accompanying symptoms point to potentially serious conditions. The American College of Emergency Physicians recommends seeking immediate care if a fever occurs alongside any of the following:
- Stiff neck that resists movement, especially combined with severe headache and light sensitivity (signs of meningitis)
- Rash with small bleeding spots under the skin, which can indicate a bloodstream infection
- Confusion, altered speech, extreme sleepiness, or difficulty waking
- Seizures or convulsions
- Difficulty breathing
- Severe abdominal pain with nausea or vomiting
- Strange or unusual behavior
A high-grade fever above 103°F in an adult that doesn’t respond to fever-reducing medication also warrants medical evaluation, as does any fever lasting more than three days without a clear cause. Persistent, unexplained fevers can occasionally signal conditions like certain cancers or deep-seated infections that aren’t obvious from the outside.
Managing a Fever at Home
For a typical fever from a cold or flu, the priority is staying hydrated. Your body loses more water through sweating and faster breathing when your temperature is elevated, and dehydration can make you feel significantly worse. Water, broth, and drinks with electrolytes all help. Light, loose clothing and a comfortable room temperature let your body regulate heat more effectively than piling on heavy blankets, even if the chills make you want to bundle up.
Over-the-counter fever reducers can bring relief when the discomfort is interfering with sleep or daily function. You don’t need to treat every fever, though. If your temperature is in the low-grade range and you feel reasonably okay, letting it do its job is a valid choice. Rest matters more than most people realize: your immune system consumes enormous amounts of energy during an infection, and sleep is when many repair and defense processes peak.