What Is a Low-Grade Fever?

A low-grade fever is usually defined as a body temperature between 99 F (37.2 C) and 100.3 F (37.9 C). Some healthcare providers define a low-grade fever as a body temperature between 100 F (37.8 C) and 102 F (38.9 C).

A normal body temperature is about 98.6 F (37 C), with slight variations also considered normal in healthy people. These minor changes may be due to several factors, such as whether the temp is taken orally or rectally, or in the morning or the evening.

This article explains low-grade fever symptoms associated with a number of infections or other possible causes. It offers tips on how to manage a fever and how to know when to call a healthcare provider.

Low-Grade Fever Symptoms

Outside of an elevated body temperature in the range described above, some people with a low-grade fever won’t have any noticeable symptoms. Others may experience:

• Headache
• Fatigue
• Muscle aches
• Sweating
• Chills
• Loss of appetite
• Lower urine output

Most low-grade fevers run their course in a few days and don’t require medical attention.

A low-grade fever is generally not cause for concern unless it is persistent and/or accompanied by other, more serious symptoms, like vomiting or pain.

What Causes a Low-Grade Fever?

A fever—even a low-grade one—is a sign that something is wrong with your body. It indicates that your immune system is mounting a defense against a foreign invader, be it a virus, bacterium, or other matter.

Experts aren’t exactly sure why the body responds to infections and illnesses with a raised body temperature, but they think some disease-causing germs are less likely to thrive in higher temperatures.

Experts also theorize that increased body temperatures might better enable certain immune cells to seek out and destroy whatever is attacking the body.

Infections

Both viral and bacterial infections can cause low-grade fevers. Some common culprits include:

• Rhinoviruses (aka the common cold)
• Flu
• Measles
• Mumps
• Chickenpox (varicella-zoster virus)
• Rubella (also called German measles)
• COVID-19
• Urinary tract infections
• Pneumonia
• Ear infections
• Strep throat
• Cellulitis, an infection of the skin and the tissues around it
• Sinus infections

Other common causes include:

• Infectious mononucleosis (mono), a highly contagious illness that usually affects teens and young adults. It’s often caused by infection with the Epstein-Barr virus.
• Viral gastroenteritis (otherwise known as the stomach flu). Examples of these include the rotavirus, common in babies, and norovirus, which generally affects adults and is highly contagious.
• Escherichia coli (E. coli), a bacterial infection. Some strains of E.coli that live in the intestines are harmless, while others can produce serious illness.
  

Non-Infectious Illnesses

Certain autoimmune diseases, such as rheumatoid arthritis and lupus, can display a low-grade fever. In fact, up to 86% of people with lupus experience fever, usually a low-grade one.

Some malignant tumors (cancers) can cause cellular changes in the body that produce fevers. What’s more, the chemotherapy used to treat some cancers can destroy your body’s disease-fighting white blood cells, making you more susceptible to infections and the fevers that come with them. 

Other Causes

Other causes of low-grade fever include:

• Stress: Stress causes low-grade fever in some people. Young women, notably, respond to stress with higher core body temperatures. This is known as psychogenic fever.
• Vaccines: Low-grade fever is a common side effect of many vaccines, including the flu shot and childhood immunizations. The fever is a sign your body is building immunity to the disease.
• Sensitivity to certain drugs: Some fevers can be attributed to adverse reactions to certain drugs, including antibiotics and drugs that treat seizures. These "drug fevers" have been seen during treatment of COVID-19, and with certain cancer treatments.
• Menstruation: Research shows an increase in basal body temperature of between 0.3 C and 0.7 C (roughly about 0.4 F) during and immediately after ovulation.
  

Taking a Temperature

You may look flushed and feel warm when you have a fever, but the only way to accurately know your temperature is to measure it with a thermometer.

Digital thermometers, placed under the tongue or with the tip inserted into the anus (rectal temperature-taking is recommended for babies and very young children) offer the most reliable readings.

Thermometers placed under the armpit (called axillary) can give inaccurate readings. The same is true for plastic strip, smartphone temperature apps, and pacifier thermometers.

Thermometers that scan the forehead or are placed inside the ear (called tympanic) are OK to use, but don't use a tympanic thermometer if you're taking the temperature of baby younger than 3 months of age or anyone with an ear infection.

Treatment

The general rule of thumb is to treat a low-grade fever only if it’s causing you discomfort. And some experts say that doesn’t happen until a temperature hits 102 F (38.9 C) to 103 F (39.4 C) and higher. 

There’s also a pretty vocal school of thought that says when you suppress a fever, you also suppress the immune system’s production of disease-busting white blood cells, thereby prolonging the illness.

When you want to treat a low-grade fever, Tylenol (acetaminophen) and Advil (ibuprofen) are good choices. Follow the package directions for proper dosage, or ask your healthcare provider.

Aspirin is another option, but never give aspirin to children or teens unless instructed by your healthcare provider. Aspirin use in this age group has been associated with a serious and potentially deadly disorder called Reye’s syndrome.

Self-help measures include:

• Staying hydrated, because fevers can cause dehydration as your body loses water through sweating or rapid breathing
• Resting
• Wearing light clothing
• Eating light, easy-to-digest foods, such as broth-based soups

When to Should I Be Concerned About a Low-Grade Fever?

Call your healthcare provider or make a trip to the emergency room if any of the following apply to you or someone you’re caring for:

• You have a baby under 3 months of age with any fever. Don’t give fever-reducing medicines, such as Infants’ Tylenol, without consulting your healthcare provider.
• You have a fever and a stiff neck. This is a symptom of a life-threatening condition called meningitis, which affects the brain and spinal cord.
• You have a fever and a rash. The two combined can indicate some serious conditions, such as toxic shock syndrome and Rocky Mountain spotted fever. 
• You’re feeling dehydrated. Signs include dry mouth, dry skin, sunken eyes and dark urine.
• Your child has a fever and experiences a seizure. Fevers—especially ones that rise quickly—can sometimes trigger what’s known as febrile seizures in young children. 
• You feel very sick. For example, you're unable to get out of bed.
• You have a fever and severe pain or inflammation anywhere in your body.
• You have pain when you urinate or your urine is foul-smelling. This may be a sign of a urinary tract infection (UTI) that needs treatment with an antibiotic.
• You’re disoriented. You may feel dizzy, lightheaded, or even experience hallucinations.
• It won't go away. A fever that lasts more than two days is a sign you need to be evaluated.
  

Fevers of any kind can be stressful. But it’s important to remember that most low-grade fevers are harmless and run their course in a few days or less. Call your healthcare provider if you’re worried about symptoms or simply want to ask for advice and reassurance.