A low-grade fever is a body temperature that’s slightly above normal but below what most doctors consider a full fever. Many healthcare providers define it as an oral temperature between 99.5°F (37.5°C) and 100.3°F (37.9°C), though there’s no universally standardized range. A full fever typically starts at 100.4°F (38°C), which is the threshold the CDC uses for screening purposes.
The Temperature Range and Why It Varies
Your body temperature isn’t a fixed number. It naturally fluctuates throughout the day, dipping lowest in the early morning (around 4 to 6 a.m.) and peaking in the early evening. This daily swing can be as much as 0.5°F to 1.9°F, which means a reading of 99.1°F at 5 p.m. might be completely normal for you, while the same reading at 6 a.m. could signal something is off.
Where you take your temperature also matters. Rectal and ear thermometers read about 0.5 to 1°F higher than oral thermometers, while forehead and armpit thermometers read about 0.5 to 1°F lower. So if you’re using a forehead scanner and it reads 99°F, the equivalent oral temperature would be closer to 99.5°F. Keep this offset in mind when comparing your reading to any published fever threshold.
What Causes a Low-Grade Fever
The most common cause is a viral infection. Colds, in particular, often produce a low-grade fever that lingers for several days rather than spiking high. Viral bronchitis and viral pneumonia can do the same. Your immune system raises your body temperature intentionally to make the environment less hospitable to invading pathogens, which is why low-grade fevers are generally considered a productive part of your body’s defense.
Bacterial infections can also be responsible. Urinary tract infections are a frequent culprit, sometimes producing a persistent low-grade fever as the only noticeable symptom in both children and adults. Tuberculosis is another bacterial cause, known for producing low-grade fevers that worsen at night and lead to drenching night sweats.
Non-Infectious Causes
Not every low-grade fever means you’re fighting a bug. Several other triggers can raise your temperature slightly:
- Medications: A new drug can cause a low-grade fever about 7 to 10 days after you start taking it. Antibiotics are the most frequent offenders, but cardiovascular drugs, anti-seizure medications, anti-inflammatory drugs, and even some sleep and pain medications can do this. If the timing lines up, your medication is worth mentioning to your doctor.
- Autoimmune conditions: Rheumatoid arthritis, multiple sclerosis, and other chronic inflammatory diseases can cause mildly elevated temperatures. In rheumatoid arthritis, the fever is thought to come from ongoing joint inflammation.
- Thyroid inflammation: A condition called subacute thyroiditis, where the thyroid gland becomes inflamed from infection, radiation, or autoimmune activity, can produce a low-grade fever.
- Chronic stress: Prolonged emotional stress can actually raise body temperature, a phenomenon called psychogenic fever. It’s most common in people with conditions like chronic fatigue syndrome or fibromyalgia.
- Teething in babies: Infants between 4 and 7 months old may develop a mild temperature elevation while teething, though it rarely rises above the low-grade range.
In rare cases, a persistent and otherwise unexplained low-grade fever can be an early sign of certain cancers, particularly lymphomas and leukemias. This is uncommon, but it’s one reason doctors take seriously any fever that lasts weeks without a clear explanation.
Should You Treat a Low-Grade Fever?
In most cases, no. The Mayo Clinic notes that minor fevers may actually help your body fight off infection by reducing the number of microbes causing your illness. Fever-reducing medications are generally recommended only when temperatures climb above 102°F (38.9°C) or when the fever is causing significant discomfort. For a low-grade fever, rest and staying hydrated are typically enough.
For infants between birth and 12 months, Children’s Hospital Colorado considers temperatures in the 100°F to 102°F range a “helpful, good range” and advises against treating them with medication. That said, the rules change sharply for very young babies: any infant under 3 months old with a rectal temperature of 100.4°F or higher needs medical evaluation right away, regardless of how well they seem.
When a Low-Grade Fever Needs Attention
A low-grade fever by itself is rarely dangerous, but certain accompanying symptoms warrant prompt medical care. In adults, seek immediate attention if a fever of any level comes with a severe headache, stiff neck, rash, sensitivity to bright light, confusion or altered speech, persistent vomiting, difficulty breathing, chest pain, or seizures. Pain when urinating alongside a fever also points to a possible infection that needs treatment.
In children, a fever lasting longer than three days, repeated vomiting, extreme listlessness, or poor eye contact are signs to call your pediatrician. For babies between 3 and 6 months, a rectal temperature above 102°F or unusual irritability at a lower temperature warrants a call. Between 7 and 24 months, a temperature above 102°F that persists for more than a day without other symptoms should also be evaluated.
Persistent Low-Grade Fever Without a Clear Cause
When a low-grade fever keeps coming back or never fully resolves over weeks, doctors may begin investigating it as a fever of unknown origin. The current definition requires a documented temperature of 101°F (38.3°C) or higher on multiple occasions with no diagnosis after initial testing. The workup typically involves blood tests, imaging, and a careful review of all medications, since drug fever is one of the most commonly overlooked explanations. Previous definitions required a specific minimum duration of investigation, but current guidelines recognize that timeline as arbitrary and focus instead on the thoroughness of the evaluation.
If you’ve had a low-grade fever for more than two to three weeks and feel otherwise fine, it’s worth tracking your temperatures at consistent times of day using the same thermometer and the same body site. This log gives your doctor a much clearer picture than a single reading taken at a random time, especially given how much your temperature naturally varies across 24 hours.