Fevers in adults are most commonly caused by infections, but they can also result from autoimmune conditions, medications, cancers, and other triggers. A fever is generally defined as a body temperature at or above 101°F (38.3°C), though many people start feeling feverish at slightly lower temperatures. Understanding the cause matters because it determines whether a fever needs treatment or will resolve on its own.
How Your Body Creates a Fever
A fever isn’t a malfunction. It’s a deliberate response orchestrated by a temperature control center deep in your brain called the hypothalamus, which normally keeps your body at roughly 97.7–98.6°F. When your immune system detects a threat, white blood cells release signaling molecules that travel through your bloodstream and reach the hypothalamus. There, they trigger the production of a chemical messenger called prostaglandin E2, which essentially turns up your internal thermostat.
Once the set point rises, your body acts as if its normal temperature is too cold. You shiver, your blood vessels constrict, and you feel chilled, all of which generate and conserve heat until your temperature climbs to the new, higher target. This elevated temperature makes it harder for many bacteria and viruses to replicate, while also boosting certain immune functions. When the infection clears or you take a fever reducer (which works by blocking prostaglandin production), the set point drops back down and you start sweating to cool off.
Infections: The Most Common Cause
The vast majority of fevers in adults are triggered by infections. If your fever lasts four days or less, an infection is the most likely explanation. Virtually any infection can cause a fever, but the most frequent culprits fall into a few categories:
- Upper and lower respiratory infections like colds, the flu, sinusitis, bronchitis, and pneumonia. Most acute respiratory infections are viral and resolve without antibiotics.
- Gastrointestinal infections from viruses like norovirus or bacteria like Salmonella, typically accompanied by nausea, vomiting, or diarrhea.
- Urinary tract infections, which can cause fever along with painful or frequent urination. A UTI that reaches the kidneys often produces a higher fever with flank pain.
- Skin infections such as cellulitis or infected wounds, where bacteria enter through broken skin and cause redness, swelling, and warmth at the site.
Viral infections account for the majority of short-lived fevers. They typically peak within a few days and resolve within a week. Bacterial infections can also cause brief fevers, but they’re more likely to require targeted treatment. Fungal infections cause fevers less often but tend to occur in people with weakened immune systems.
Autoimmune and Inflammatory Conditions
When the immune system mistakenly attacks the body’s own tissues, the resulting inflammation can produce fevers that come and go for weeks or months. These fevers often have no obvious pattern and can be frustrating to diagnose because standard blood tests for autoimmune markers are nonspecific, sometimes showing positive results even in people with infections or cancers rather than autoimmune disease.
The autoimmune conditions most commonly linked to persistent or recurring fevers in adults include Adult Still’s disease (which causes high, spiking fevers along with joint pain and a salmon-colored rash), systemic lupus erythematosus, and various forms of blood vessel inflammation. In older adults, a condition called temporal arteritis, which inflames arteries near the temples, is a particularly notable cause of unexplained fever. Diagnosing these conditions often requires repeated testing over time, and in some cases, a trial of anti-inflammatory medication to see whether the fever responds.
Medications and Substances
Dozens of medications can raise body temperature, either as a side effect or through a dangerous reaction. Drug-induced fever is easy to overlook because it can start days to weeks after beginning a new medication, making the connection less obvious.
Stimulants like cocaine and amphetamines raise body temperature by ramping up the nervous system. Certain antidepressants can trigger serotonin syndrome, a potentially dangerous condition where excess serotonin causes high fever, agitation, and muscle rigidity, especially when multiple serotonin-affecting drugs are combined. Antipsychotic medications can cause a rare but serious reaction called neuroleptic malignant syndrome, which produces very high fevers along with severe muscle stiffness. Even common over-the-counter antihistamines can occasionally raise temperature by blocking the body’s ability to sweat.
Withdrawal from alcohol or sedatives is another overlooked cause. When someone who has been drinking heavily or taking benzodiazepines regularly stops abruptly, the rebound in nervous system activity can produce significant fevers along with tremors and confusion.
Cancer-Related Fevers
Unexplained fevers that persist for weeks, particularly when accompanied by unintentional weight loss, drenching night sweats, and fatigue, can be a sign of certain cancers. Lymphoma is one of the malignancies most closely associated with fever. The classic combination of fever, night sweats, and weight loss (known as “B symptoms”) is so characteristic that doctors specifically ask about it when lymphoma is suspected.
Other cancers that commonly cause fever include leukemia, kidney cancer, and liver cancer. Tumors can trigger fevers by releasing inflammatory substances or by causing tissue destruction that activates the immune system. Cancer-related fevers tend to be low-grade and persistent rather than the high, sharp spikes typical of bacterial infections.
Heat-Related Illness vs. True Fever
Not every high body temperature is a fever. Heat stroke, the most dangerous form of heat illness, can push body temperature to 106°F or higher within 10 to 15 minutes. The critical difference is that in heat stroke, the body’s cooling system has failed. The hypothalamus hasn’t raised the set point; instead, the body simply can’t shed heat fast enough. Sweating often stops, which is a key warning sign.
This distinction matters for treatment. A true fever responds to fever-reducing medications because those drugs lower the hypothalamic set point. Heat stroke does not respond to these medications because the set point was never raised. Heat stroke requires rapid external cooling and is a medical emergency.
Fever of Unknown Origin
When a fever of 101°F or higher persists across multiple episodes and standard testing fails to reveal a cause, it falls into a category called fever of unknown origin. This diagnosis essentially means the fever is real and documented, but initial bloodwork, imaging, and cultures haven’t explained it. The three most common categories behind these stubborn fevers are infections that are difficult to detect (like abscesses or tuberculosis), autoimmune conditions, and cancers.
Reaching a diagnosis often takes time. Doctors may need to repeat blood tests over several weeks, order specialized imaging, or perform biopsies. In some cases, the fever eventually resolves on its own without a cause ever being identified.
Warning Signs That Need Immediate Attention
Most fevers in adults are harmless and self-limiting. However, certain symptoms alongside a fever point to potentially serious conditions. A stiff neck combined with severe headache and sensitivity to light can indicate meningitis. Mental confusion, altered speech, or strange behavior during a fever may signal an infection affecting the brain or dangerously high body temperature. Persistent vomiting, difficulty breathing, chest pain, and seizures during a fever all warrant emergency evaluation.
A rash appearing with a fever also deserves prompt attention, as it can indicate conditions ranging from allergic drug reactions to serious infections like meningococcemia. Pain with urination alongside a high fever suggests a kidney infection that may need antibiotics to prevent it from spreading to the bloodstream.