Fever has dozens of possible causes, but the vast majority fall into a few categories: infections, inflammatory conditions, medications, and rarely, cancer. A fever is generally defined as a body temperature above 100.4°F (38.0°C), though normal body temperature fluctuates throughout the day and varies slightly by age and measurement method.
How Your Body Creates a Fever
Fever isn’t something that happens to you passively. Your body actively raises its own temperature as a defense mechanism. When your immune system detects a threat, such as bacteria or a virus, specialized cells called macrophages release signaling molecules (cytokines) into the bloodstream. These signals travel to a region at the base of your brain that acts as your internal thermostat.
Once those signals arrive, your brain essentially turns up the dial. It triggers a cascade that raises your body’s temperature set point, much like adjusting a thermostat in your house. Your body then works to reach that new, higher temperature through two main strategies: constricting blood vessels near the skin to trap heat, and triggering shivering to generate more heat. That’s why you often feel cold and shaky at the start of a fever, even though your temperature is climbing.
Infections: The Most Common Cause
The overwhelming majority of fevers come from infections. Your body raises its temperature because many pathogens reproduce less efficiently in warmer conditions, and certain immune functions work better at higher temperatures.
Upper respiratory infections are the most frequently evaluated acute illness in outpatient settings. These are typically caused by viruses like rhinovirus, influenza, parainfluenza, and respiratory syncytial virus (RSV). Most of these fevers are low-grade and resolve on their own within a few days as the immune system clears the virus.
Bacterial infections tend to produce higher fevers. Strep throat, caused by group A streptococci, produces fever in over 90% of cases. Bacterial causes of food poisoning, including Salmonella and Shigella, commonly trigger high fevers alongside gastrointestinal symptoms. Bacterial meningitis, a medical emergency, causes fever along with a stiff neck and sensitivity to light.
Urinary tract infections, ear infections, sinus infections, pneumonia, and skin infections like cellulitis are other everyday bacterial causes of fever. Viral gastroenteritis (“stomach flu”), mononucleosis, and COVID-19 are common viral culprits as well.
Autoimmune and Inflammatory Conditions
When no infection is present, recurring or persistent fevers sometimes point to an inflammatory condition. In these disorders, the immune system generates the same signaling molecules that normally fight infections, but without an actual pathogen to target. The result is periodic fevers that can be confusing and difficult to diagnose.
Lupus, rheumatoid arthritis, and inflammatory bowel disease can all cause fevers during flares. A group of conditions called autoinflammatory syndromes specifically cause recurrent fever episodes. Familial Mediterranean fever, one of the most recognized, produces attacks of fever and intense abdominal or chest pain that resolve within 12 to 72 hours. Another condition called TRAPS causes fever in more than 95% of patients, along with migrating muscle pain, abdominal pain, and rashes.
Children sometimes develop a condition called PFAPA syndrome, which causes clockwork-like fever episodes alongside mouth sores, sore throat, and swollen neck glands. These episodes recur every few weeks and are not caused by infection, though they can easily be mistaken for repeated bouts of strep throat.
Cancer-Related Fevers
Certain cancers can cause fever directly, either by triggering an immune response or by producing the same signaling molecules your body uses during infection. Lymphomas are the most well-known example. About 25% of people with Hodgkin’s lymphoma experience what are called “B symptoms”: fever, drenching night sweats, and unexplained weight loss.
Up to 20% of people with kidney cancer present with fever, and the likelihood of fever correlates with tumor size. Leukemias, liver cancer, ovarian cancer, and certain brain tumors can also produce unexplained fevers. A fever caused by cancer typically persists for weeks, doesn’t respond to antibiotics, and occurs alongside other symptoms like unintentional weight loss or fatigue.
Medications That Trigger Fever
Drug-induced fever is more common than most people realize, and it can develop even after you’ve been taking a medication for weeks without issue. The fever typically resolves within 48 to 72 hours after stopping the responsible drug.
The list of medications known to cause fever is long. Some of the most frequently implicated categories include:
- Antibiotics: penicillins, cephalosporins, sulfonamides, and vancomycin
- Seizure medications: carbamazepine and phenytoin
- Heart and blood pressure drugs: certain blood thinners, diuretics, and heart rhythm medications
- Anti-inflammatory drugs: ibuprofen, naproxen, and celecoxib (paradoxically, since these are also used to treat fever)
- Chemotherapy drugs: multiple classes
Drug fever is tricky because there’s no specific test for it. It’s usually diagnosed by ruling out other causes and observing whether the fever disappears after the medication is discontinued.
Fever After Vaccination
A low-grade fever after a vaccine is a normal immune response, not a sign of illness. It means your immune system is reacting to the vaccine and building protection. These fevers are typically mild and short-lived, resolving on their own within two to three days. They usually begin within a day or two of the shot. Nearly all routine vaccines, from flu shots to childhood immunizations, list low-grade fever as a common and expected side effect.
Other Causes Worth Knowing
Heat-related illness, including heat exhaustion and heatstroke, raises body temperature but through a different mechanism than true fever. Instead of the brain resetting its thermostat, the body simply can’t cool itself fast enough. The distinction matters because the treatment approach is different.
Blood clots, particularly deep vein thrombosis or pulmonary embolism, can cause low-grade fevers. So can major tissue injury from surgery, burns, or trauma. Hyperthyroidism occasionally raises body temperature. Even intense physical exercise can temporarily push your temperature above the fever threshold.
When a Fever Lasts Too Long
Most fevers resolve within a few days as the underlying cause clears. When a fever of 101°F (38.3°C) or higher persists for three weeks or more without a clear explanation despite thorough testing, it meets the medical definition of “fever of unknown origin.” This classification triggers a more extensive workup, since prolonged fevers are more likely to stem from atypical infections, autoimmune conditions, or malignancies than from routine viral illness.
Red Flags in Children
In young children, the fever itself is less important than how the child looks and behaves. A child with a moderate fever who is playing and drinking fluids is generally less concerning than a child with a lower fever who is limp or unresponsive. That said, certain signs alongside a fever call for immediate medical attention:
- In infants under 3 months: any fever of 100.4°F (38.0°C) or higher, regardless of how the baby appears
- Difficulty breathing: fast or labored breathing, chest pulling inward, or blue-tinged lips
- Stiff neck: resistance to bending the neck forward, a possible sign of meningitis
- Rash that doesn’t fade when pressed: especially purple or red spots, which may indicate a serious bacterial infection
- Seizure: uncontrollable shaking or body stiffening, known as a febrile seizure
- Severe dehydration: dry mouth, no tears when crying, very few wet diapers, or a sunken soft spot on a baby’s head
- Extreme sleepiness: difficulty waking the child or lack of response to voice and touch
A fever lasting more than five days in a child, even if it’s not particularly high, generally warrants medical evaluation to check for an underlying cause that isn’t resolving on its own.