What Is a Low-Grade Fever?

Most medical experts define a low-grade fever as a body temperature between 99 F and 100.3 F. Others use a less narrow definition, referring to a low-grade fever as a body temperature ranging from 100 F to 102 F.

Your body normally maintains a temperature of about 98.6 F. Research shows that even in healthy people, normal body temperatures can vary by as much as 1.8 degrees F.

Normal body temperature can fluctuate based on several factors. Some of those factors include how your temperature is taken (for example, orally or rectally), what time of day it’s taken (your temperature tends to be higher in the evening than in the morning), and where a person who menstruates is in their menstrual cycle.

Symptoms

Outside of an elevated body temperature, 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

Causes

A fever, even a low-grade one, is a sign that something is wrong with your body. A fever 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. They also theorize that increased body temperatures might better enable certain immune cells to seek out and destroy whatever is attacking the body.

What are the frequent causes of low-grade fevers?

Infections

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

• Rhinoviruses (aka the common cold)
• Flu
• 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.
• 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
• Escherichia coli (E. coli): While some strains of E.coli, a bacterium that lives in the intestines, are harmless, others can produce serious illness.
• Sinus infections
• Infectious mononucleosis (mono): This is a highly contagious illness, usually affecting teens and young adults, that’s often caused by infection with the Epstein-Barr virus.

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: Some people, particularly young women, 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. Fever can occur in up to 10% of children receiving routine childhood vaccines and up to 70% when receiving multiple vaccines. 
• Sensitivity to certain drugs: Up to 7% of fevers can be attributed to adverse reactions to certain drugs, including antibiotics and drugs that treat seizures. This type of fever is called a drug fever.

Diagnosis

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

Digital thermometers, placed used 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 (change the batteries periodically; weak batteries can cause inaccurate 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.

One thing you shouldn’t rely on when trying to figure out if a fever is present is touch. It can miss or underestimate fevers in up to 40% of people, even when the fever is as high as 102 F.

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 to 103 F 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, acetaminophen (Tylenol) and ibuprofen (Advil) are good choices (read 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 doctor. 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: Fevers can cause your body to lose water (via things like sweating or rapid breathing), increasing the chances you may become dehydrated.
• Resting
• Wearing light clothing
• Eating light, easy-to-digest foods, such as broth-based soups

When to Call the Doctor

Most low-grade fevers run their course in a few days and don’t require medical attention. But definitely 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 doctor).
• 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.
• 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.
• Your fever lasts more than two days.

Prevention

The best way to prevent fevers is to prevent the infections that cause them:

• Wash your hands regularly with soap and water for 20 seconds—particularly after you use the bathroom and change diapers and before you prepare food or eat.
• To help prevent urinary tract infections, urinate before and after sex, drink plenty of fluids, and wipe from front to back after a bowel movement.
• Stay up to date on immunizations. Yes, certain vaccines can cause a low-grade fever in some people. But that can be a small price to pay for the protection they offer from severe or potentially life-threatening diseases.
• Avoid touching your eyes, mouth, and nose, which are prime entry points for germs.

A Word From Verywell

Fevers of any kind can be stressful. But it’s important to remember that most mild fevers are harmless and run their course in a few days or less. If your symptoms are bothersome, you can try over-the-counter fever reducers. Don’t hesitate, though, to call your doctor if you’re worried about any of your symptoms or you simply want to ask for advice and reassurance.