A low-grade fever is a body temperature slightly above normal, generally between 99.5°F (37.5°C) and 100.3°F (37.9°C) when measured orally. It signals that your immune system is responding to something, but the cause ranges widely, from a passing viral infection to an underlying condition that needs attention. Understanding the most common triggers can help you figure out whether yours is routine or worth investigating.
How Your Body Creates a Fever
Fever isn’t a malfunction. It’s a deliberate response controlled by the hypothalamus, the part of your brain that acts as a thermostat. When your immune system detects a threat like bacteria, viruses, or damaged tissue, it releases signaling molecules called cytokines into the bloodstream. The key players are interleukin-1 (IL-1), tumor necrosis factor (TNF), and interleukin-6 (IL-6).
These cytokines travel to a specific region near the hypothalamus, where they trigger production of a lipid called prostaglandin E2. This compound essentially turns the thermostat dial up, raising your body’s target temperature. Your body then works to reach that new set point through shivering, constricting blood vessels near the skin, and conserving heat. A low-grade fever means the set point has been raised only slightly, enough to help your immune system work more efficiently without the intensity of a full-blown fever.
Infections: The Most Common Cause
The vast majority of low-grade fevers come from infections, and most of those are viral. Common colds, upper respiratory infections, sinus infections, and mild stomach bugs routinely produce temperatures in the low-grade range. These fevers typically resolve on their own within a few days as your body clears the virus.
Certain infections are particularly known for producing low-grade fevers that linger. Urinary tract infections often cause a mild fever alongside burning during urination and frequent urges to go. Epstein-Barr virus (the cause of mono) and cytomegalovirus can produce weeks of low-grade fever along with fatigue and swollen lymph nodes, especially in adults. Brucellosis, a bacterial infection transmitted through unpasteurized dairy or animal contact, often causes a low-grade fever that waxes and wanes. Tuberculosis, particularly when it affects organs outside the lungs, is a well-recognized cause of prolonged unexplained fever. Even typhoid fever, caused by Salmonella, typically starts as a low-grade fever in its first week before climbing higher.
Autoimmune and Inflammatory Conditions
When your immune system mistakenly attacks your own tissues, the resulting inflammation produces the same cytokines that cause fever during an infection. Lupus is one of the most common autoimmune causes. Fever occurs in 36% to 86% of lupus patients and can be one of the earliest symptoms, sometimes appearing before joint pain or skin changes become obvious.
Rheumatoid arthritis, inflammatory bowel disease, and vasculitis (inflammation of blood vessels) can all produce persistent low-grade fevers. Sarcoidosis, a condition where clusters of inflammatory cells form in the lungs and other organs, is another culprit. In these conditions, the fever tends to come and go alongside flares of other symptoms like joint stiffness, fatigue, or digestive problems. If you have a low-grade fever that keeps returning without an obvious infectious cause, an underlying inflammatory condition is one of the possibilities your doctor will consider.
Cancer-Related Fever
Persistent, unexplained low-grade fever can sometimes be an early sign of certain cancers, particularly lymphomas and leukemias. In non-Hodgkin lymphoma, fever is one of the “B symptoms,” a trio that also includes drenching night sweats and unexplained weight loss. The presence of B symptoms affects both prognosis and treatment decisions.
Cancer-related fevers happen because tumor cells and the immune cells responding to them release the same inflammatory cytokines involved in infection. The fever is often low-grade and persistent rather than dramatic. It may come with fatigue, swollen lymph nodes, or weight loss that can’t be explained by changes in diet or activity. To be clear, cancer is a far less common cause of low-grade fever than infection. But a fever lasting more than three weeks without an obvious explanation warrants a thorough workup.
Medications That Trigger Fever
Drug-induced fever is more common than most people realize, and it can be tricky to identify because the timing doesn’t always line up neatly with starting a new medication. Antibiotics are among the most frequent offenders, particularly penicillin-type drugs, cephalosporins, and sulfa antibiotics like trimethoprim-sulfamethoxazole. Anticonvulsants such as phenytoin and carbamazepine are also well-documented causes.
The list extends to heart medications (including certain blood thinners and blood pressure drugs), anti-inflammatory drugs like ibuprofen and naproxen, and immunosuppressants used after organ transplants. Even allopurinol, commonly prescribed for gout, can cause a febrile response. Drug fever typically resolves within 48 to 72 hours of stopping the offending medication, which is often how the diagnosis is confirmed.
Thyroid and Hormonal Causes
Your thyroid hormones directly influence body temperature by controlling how fast your cells burn energy. In hyperthyroidism, an overactive thyroid floods the body with excess thyroid hormone, speeding up metabolism throughout every cell. This increased metabolic rate generates more heat, which can push body temperature into the low-grade fever range. Other signs include a rapid heartbeat, unintentional weight loss, trembling hands, and feeling unusually warm when others around you are comfortable.
Hormonal shifts during the menstrual cycle also affect body temperature. Progesterone, which rises after ovulation, can raise basal body temperature by about 0.5°F to 1.0°F. This is normal and not a sign of illness, but it can push readings into the low-grade fever range if your baseline runs on the higher side.
Stress, Activity, and Normal Variation
Not every low-grade temperature reading signals a problem. Body temperature naturally fluctuates throughout the day, running lower in the morning and peaking in the late afternoon or early evening. Vigorous exercise, hot weather, heavy clothing, and even eating a large meal can temporarily raise your reading. Emotional stress and anxiety activate your sympathetic nervous system, which can also nudge temperature slightly upward.
Where you measure matters, too. Rectal and ear thermometers read about 1.0°F higher than oral thermometers, while forehead thermometers read about 1.0°F lower. If you’re comparing readings across different methods, you may be seeing a measurement difference rather than a true change in body temperature.
When a Low-Grade Fever Needs Attention
A low-grade fever lasting a day or two during a cold or mild infection is generally nothing to worry about. The threshold for concern is duration and context. A fever persisting beyond three days without a clear cause deserves a call to your doctor. If a fever exceeds three weeks and remains unexplained after initial evaluation, it meets the clinical definition of a fever of unknown origin, which triggers a more systematic diagnostic process.
Certain accompanying symptoms raise the urgency. A stiff neck with pain when bending your head forward, a new rash, confusion or altered speech, persistent vomiting, difficulty breathing, or pain when urinating alongside a fever all warrant prompt medical attention. Unexplained weight loss, drenching night sweats, or growing fatigue paired with a persistent low-grade fever also call for evaluation, as these patterns can point to autoimmune disease, chronic infection, or malignancy. For infants under three months, any rectal temperature of 100.4°F or higher is considered an emergency regardless of how the baby appears.