A fever happens when your brain’s internal thermostat, located in a region called the hypothalamus, raises your body’s temperature set point above its normal range. Most healthcare providers define a fever as a temperature of 100.4°F (38°C) or higher when measured by mouth, while readings between 99.5°F and 100.3°F are generally considered a low-grade fever. Infections are the most common trigger, but fevers can also result from medications, vaccines, autoimmune conditions, and other non-infectious causes.
How Your Body Creates a Fever
Your hypothalamus acts as a thermostat, constantly monitoring your blood temperature and adjusting it through sweating, blood vessel dilation, and metabolic changes. When your immune system detects a threat, it releases signaling molecules called pyrogenic cytokines. These chemicals travel to the hypothalamus and trigger the production of a compound called PGE2, which essentially turns the thermostat up. Your body then responds as if it’s too cold: you shiver, your blood vessels constrict to conserve heat, your metabolism speeds up, and your temperature climbs.
This is different from overheating due to hot weather or intense exercise. In those cases, your thermostat hasn’t changed. Your body is simply overwhelmed and can’t cool itself fast enough. That’s called hyperthermia, and it’s treated differently from a fever. Fever-reducing medications won’t help with heat stroke because the underlying mechanism is completely different.
Infections: The Most Common Cause
Bacteria, viruses, fungi, and parasites all trigger fevers. When your immune cells encounter these foreign invaders, they kick off the chain reaction described above. The fever itself serves a purpose: elevated body temperature slows pathogen replication and boosts immune cell activity. Fungal infections are a particularly clear example, since many fungi grow best at temperatures below normal human body temperature, making fever an effective defense.
Common infections that cause fever include the flu, COVID-19, strep throat, urinary tract infections, sinus infections, pneumonia, and stomach bugs. The fever pattern varies. A viral cold might produce a low-grade fever for a few days, while a bacterial infection like strep can push temperatures higher and more abruptly. Ear infections, skin infections, and even dental abscesses can all raise your temperature if the immune response is strong enough.
Vaccines and Post-Vaccination Fever
Vaccines work by training your immune system to recognize a specific pathogen, so it’s no surprise that they can trigger the same fever response as an actual infection. Most post-vaccination fevers appear within a week of the shot and resolve within one to two days.
How common this is depends on the vaccine. The pneumococcal vaccine given to infants causes fever in roughly 24 to 35 out of 100 recipients. The rotavirus vaccine triggers fever in about 17 to 20 out of 100 babies. For older children and adults, rates tend to be lower: the HPV vaccine causes fever in about 6 to 7 out of 100 people, and hepatitis B vaccines cause it in 1 to 10 out of 100, depending on the specific product. These fevers are a sign that the immune system is responding, not a sign that something has gone wrong.
Medications That Can Cause Fever
Some medications trigger fever as a side effect, a phenomenon known as drug fever. Antibiotics are the most common culprits. In one large French database study, 68% of drug fever cases involved an antibiotic or anti-infective medication. The reaction can stem from a hypersensitivity response, where your immune system reacts to the drug itself rather than to an infection.
Beyond antibiotics, certain seizure medications like carbamazepine and phenytoin are known to cause drug fever. So are some antipsychotics, antidepressants, and thyroid medications. Drug fever can be tricky to recognize because if you’re already taking an antibiotic for an infection, a persistent fever might look like the infection isn’t improving when the medication itself is actually the problem. The fever typically resolves once the offending drug is stopped.
Autoimmune and Inflammatory Conditions
Your immune system doesn’t need an infection to trigger a fever. In autoimmune diseases like lupus and rheumatoid arthritis, the immune system mistakenly attacks healthy tissue, producing the same inflammatory signals that raise body temperature. These fevers tend to be recurring and low-grade, often appearing alongside joint pain, fatigue, or skin changes.
Certain cancers, particularly lymphomas and leukemias, can also cause persistent fevers. Autoinflammatory diseases, a related group of conditions where the body’s innate immune system misfires, produce episodes of fever that come and go in patterns. Because these symptoms overlap with infections and even cancers, unexplained recurring fevers often require careful evaluation to pin down the cause.
Fever Thresholds That Matter
For adults, a temperature over 104°F (40°C) warrants a call to your doctor. Seek immediate medical help if a fever comes with a seizure, confusion, loss of consciousness, a stiff neck, trouble breathing, severe pain anywhere in the body, or painful urination with foul-smelling urine.
The rules are stricter for babies. Any fever in an infant younger than 3 months old requires a call to your pediatrician, regardless of how high or low it is. For babies 3 to 6 months old, a temperature of 100.4°F or higher, or any fever where the baby seems unusually sluggish or irritable, is reason to call. Between 6 and 24 months, a fever above 100.4°F that lasts more than one day needs medical attention. Young infants have immature immune systems, so a fever can signal a serious infection that older children and adults would fight off more easily.
Why Your Body Bothers With Fever
Fever feels miserable, but it evolved as a defense mechanism. Higher body temperatures directly slow the replication of many bacteria and viruses, buying your immune system time to mount a stronger response. Historically, controlled heat therapy was even used as a medical treatment. Doctors once deliberately induced fevers to treat syphilis, and heat therapy showed effectiveness against poliovirus.
This is why mild to moderate fevers don’t always need to be treated with medication. Bringing a fever down with over-the-counter remedies can make you more comfortable, but it doesn’t speed up recovery from the underlying illness. The fever itself is doing work. That said, very high fevers or fevers that cause significant discomfort, dehydration, or difficulty sleeping are worth managing for quality of life alone.