Fever has dozens of possible causes, but most fall into a few major categories: infections, inflammatory conditions, medications, cancers, and environmental heat exposure. An oral temperature of 100°F (37.8°C) or higher is generally considered a fever. Understanding what triggers that rise in temperature can help you figure out what your body is responding to and whether you need to act on it.
How Your Body Creates a Fever
A fever isn’t something that happens to you passively. Your body actively generates it. When your immune system detects a threat, such as a virus or bacteria, immune cells release signaling molecules called cytokines. These molecules travel to the brain’s temperature control center, the hypothalamus, and essentially reset your internal thermostat to a higher level. Once that set point rises, your body kicks into gear: muscles shiver to produce heat, blood vessels near the skin constrict to reduce heat loss, and you feel cold even though your temperature is climbing. This is why you get the chills at the start of a fever.
This process is fundamentally different from overheating due to hot weather or exercise, where your body temperature rises against its will. In a true fever, your body is deliberately raising the temperature because the immune system has requested it. That distinction matters because the two situations call for different responses.
Infections: The Most Common Cause
The vast majority of fevers are caused by infections. Virtually any type of pathogen can trigger one.
- Viruses are the single most frequent culprit. The common cold, flu, COVID-19, stomach bugs, and respiratory infections all routinely cause fevers. Most viral fevers resolve on their own within a few days as your immune system clears the infection.
- Bacteria cause fevers in conditions like strep throat, urinary tract infections, pneumonia, skin infections like cellulitis, and tuberculosis. Bacterial infections often need antibiotic treatment, and the fever typically breaks once the infection is under control.
- Fungi can cause fevers, though this is less common in healthy people. Fungal infections are more likely to cause persistent or unexplained fevers in people with weakened immune systems.
- Parasites like the organism that causes malaria are a major source of fever worldwide, particularly in tropical regions. Malaria characteristically produces cycles of high fever that spike and then drop.
If you have a fever with an obvious infection, like a sore throat or cough, the cause is usually straightforward. Fevers that appear without a clear source are more likely to need investigation.
Autoimmune and Inflammatory Conditions
Your immune system doesn’t need an actual infection to trigger a fever. In autoimmune diseases, the immune system mistakenly attacks healthy tissue, producing the same inflammatory signals that normally fight pathogens. This means conditions like rheumatoid arthritis, lupus, and inflammatory bowel disease can all cause recurring or low-grade fevers, sometimes as one of the earliest symptoms before other signs appear.
A related group of conditions, called autoinflammatory diseases, involves a different branch of the immune system misfiring. These tend to cause periodic fevers that come and go in patterns, sometimes lasting days at a time before disappearing for weeks. The fevers themselves can be the primary symptom, which makes these conditions tricky to identify.
Medications That Trigger Fever
Some medications cause fever as a side effect, a phenomenon known as drug fever. Antibiotics are the most frequently reported cause, with penicillins and cephalosporins among the most common offenders. But the list extends well beyond antibiotics to include seizure medications, blood thinners, thyroid hormone replacements, antidepressants, and antipsychotics.
The timing varies widely. Some drugs cause fever within hours of the first dose. For antibiotics, the median onset is about 2 days, though it can take over a week. Anti-tuberculosis medications cause fever within 3 weeks of starting treatment in most cases. The challenge with drug fever is that it often mimics the infection the medication was prescribed to treat, leading to a frustrating cycle where the “cure” looks like the disease getting worse. If a new fever develops after starting a medication, the medication itself deserves consideration as the cause.
Some drugs raise body temperature through a different mechanism entirely. Rather than triggering an immune response, they interfere with your body’s ability to regulate heat. Certain antipsychotics and antidepressants can do this, as can stimulant medications.
Cancer-Related Fevers
Persistent or recurring fevers that don’t have an obvious explanation can sometimes be a sign of cancer. Blood cancers like lymphoma and leukemia are the types most commonly associated with unexplained fevers, though solid tumors can cause them too. These fevers happen because tumor cells release the same inflammatory cytokines that infections do, or because the immune system’s response to the cancer generates those signals. Night sweats, unintentional weight loss, and fatigue often accompany cancer-related fevers.
A fever alone is rarely the only sign of cancer, but a fever that persists for weeks without a clear infectious cause, especially alongside weight loss or drenching night sweats, warrants thorough evaluation.
Vaccines
Fever after vaccination is common and expected. It’s a sign your immune system is responding to the vaccine, not a sign that something has gone wrong. Most post-vaccine fevers appear within a week and resolve quickly.
For mRNA COVID-19 vaccines, fever typically shows up 1 to 2 days after the shot and lasts about a day, affecting roughly 10 to 30 out of every 100 recipients depending on the specific vaccine and age group. The MMR vaccine has a wider window: fever can appear anytime in the first 14 days, occurring in about 2 out of every 100 recipients. Post-vaccine fevers are almost always mild and self-limiting.
Heat Exposure vs. True Fever
Not every elevated body temperature is a fever. When your body overheats because of external conditions, like exercising in extreme heat or being stuck in a hot environment, that’s hyperthermia. The key difference is that your brain’s thermostat hasn’t been reset. Your body is trying to cool down but can’t keep up with the heat load. In a true fever, your body is actively working to raise its temperature.
This distinction has practical implications. Fever-reducing medications work by resetting the thermostat back to normal. They won’t help with heat-related illness because the thermostat was never changed. Heatstroke requires physical cooling: getting out of the heat, applying cold water, and seeking emergency care.
Fever of Unknown Origin
Sometimes a fever defies explanation despite thorough testing. When a temperature of 101°F (38.3°C) or higher persists on multiple occasions and initial testing doesn’t reveal a cause, it’s classified as a fever of unknown origin. The most common eventual diagnoses fall into three buckets: undetected infections (often deep abscesses or bone infections that don’t show up on basic tests), autoimmune or inflammatory diseases, and cancers. In some cases, no cause is ever identified, and the fever eventually resolves on its own.
Temperature Thresholds Worth Knowing
For adults, a temperature of 103°F (39.4°C) or higher is worth a call to your doctor. At 104°F (40°C) or above, you should seek medical attention promptly. For infants under 3 months, any rectal temperature of 100.4°F (38°C) or higher needs immediate medical evaluation, regardless of how the baby appears. Babies between 3 and 6 months with a rectal temperature above 102°F (38.9°C), or with a lower fever but unusual irritability or sluggishness, also need prompt attention.
At any temperature, certain accompanying symptoms signal an emergency: seizures, confusion, a stiff neck, difficulty breathing, loss of consciousness, or severe pain. These combinations suggest the fever may be part of something that needs urgent treatment, such as meningitis or sepsis.