Low Grade Fever: Causes and When to Contact a Doctor
Normal body temperature is 97.5–99.5°F (36.4–37.5ºC). It can fluctuate through the day and vary depending on the site of the measurement.
Fevers often accompany infections and are part of your body’s natural defense against them. However, there are some other causes of low grade fevers.
Low grade fevers may not require treatment if other symptoms are absent. Self-care measures, such as resting and drinking fluids, may be enough in these cases. Persistent low grade fevers may require medical care, as may those with other symptoms, such as a sore throat, an earache, a cough, or burning with urination.
Fevers can be serious in people who have weakened immune systems or chronic medical problems.
This article defines low grade fevers in adults and children. It also reviews the causes, symptoms, and treatments of this common symptom.
A low grade fever is a common symptom of COVID-19. However, viral infections of all sorts commonly cause fevers. This includes common colds and the flu. So, having a fever does not necessarily mean that you have COVID-19.
If you are in good health but develop a low grade fever, stay at home and take care of yourself. Monitor your temperature and get in touch with your doctor if it persists or worsens after 48 hours.
If you have other health conditions and develop a fever, contact your doctor right away to see if you need a COVID-19 test.
Low grade fevers can sometimes accompany serious conditions. Seek immediate medical care (call 911) for the following serious symptoms:
- pale or blue coloration of the lips or nails
- confusion or changes in consciousness
- difficulty breathing
- difficulty walking
- irritability or inconsolability in children
- neck rigidity
- a rapid heart rate
If you have other symptoms of infection, a weakened immune system, or a chronic health condition, seek prompt medical care.
Infections commonly cause a low grade fever. A low grade fever may also occur after getting a vaccine, during teething, or as a symptom of cancer or an inflammatory or autoimmune condition.
A low grade fever can also occur as a side effect of some medications.
Below is a summary of low grade fever causes.
Infectious causes of low grade fever
A fever with an infection is part of the body’s defense against the microorganism. So, almost all infections cause fevers, including infections with bacteria, viruses, fungi, and parasites. Common infectious causes include:
- respiratory tract infections, including colds, the flu, bronchitis, pneumonia, and sinus infections
- skin infections
- urinary tract infections
Other causes of low grade fever
- autoimmune conditions
- certain medications, such as antibiotics and antiseizure medications
- heat illnesses
- vaccine reactions
Also, physical activity and environmental factors — such as wearing heavy clothing or being in a high ambient temperature — can cause an increase in body temperature.
Serious or life threatening causes of low grade fever
In some cases, a low grade fever may be a symptom of a serious or life threatening condition, including:
- acute hepatitis
- heat exhaustion
- pulmonary embolism
In many cases, the cause of a low grade fever is self-limiting. This is especially true of viral infections in otherwise healthy people. The fever will resolve as the infection clears. Rest and fluids will help.
When a fever causes discomfort, certain medications can bring it down. Acetaminophen (Tylenol), ibuprofen (Advil, Motrin), and aspirin can all lower a fever. However, do not use aspirin in children under 18 years of age due to the chance of Reye’s syndrome.
Call a doctor if an infant younger than 2 months old has a fever.
The person’s age plays a role in deciding when to contact a doctor for a fever. Learn more about seeking help in the sections below.
Infants younger than 2 months
A fever in an infant this young can be dangerous. Seek immediate medical care for any increase in temperature in an infant younger than 2 months of age.
Older infants and children
- corticosteroid use
- drowsiness or fussiness
- immune system problems
- persistent vomiting or diarrhea
- symptoms of dehydration, such as a dry mouth, no tears with crying, a sunken soft spot, or fewer wet diapers than usual
- a stiff neck, severe headache, severe sore throat, or unexplained rash
- a very sick appearance
A persistent fever is another reason to contact a doctor promptly. For children younger than 2 years old, this is a fever that lasts longer than 24 hours. For older children, a fever that lasts longer than 72 hours warrants medical care. For children of all ages, fevers of 104°F (40ºC) that happen repeatedly need medical evaluation.
An adult should contact a doctor if a fever persists for longer than 48 hours. A high fever above 104°F (40ºC) is also a sign that medical care is necessary. Other serious symptoms in an adult with a fever can include:
- a change in mental status or a sudden behavioral change, such as confusion, delirium, lethargy, hallucinations, or delusions
- a headache or stiff neck
- rapid breathing or a rapid heart rate
- shortness of breath
- a rash
Digital thermometers are the best choice for taking body temperatures.
The most accurate site is the rectum for children and the mouth for adults. The forehead is the next most accurate site. Oral and ear temperatures can also be accurate, depending on the technique. The armpits are the least reliable site.
You can find digital thermometers for rectal or oral measurements. Label the thermometer “oral” or “rectal” and only use it for that site. Do not use the same thermometer for oral and rectal temperatures, even if you wash it.
There are also digital temporal thermometers available for the forehead and tympanic ones for the ear.
Here are some questions people often ask about fevers.
Is 99.6°F (37.6ºC) considered a fever?
A body temperature of 99.6°F (37.6ºC) is outside the normal range of 97.5–99.5°F (36.4 – 37.5ºC). Whether or not it indicates a fever depends on a few things. Age matters, as does where you take the body temperature readings.
For example, a reading of 99.6°F (37.6ºC) rectally in a child is normal. An oral temperature above 99.6°F (37.6ºC) in a child is a fever if the child is over 4 years old. In an adult, however, neither reading would indicate a fever.
Keep in mind that physical activity and environmental factors can raise body temperature slightly.
Is 99.2°F (37.3ºC) considered a fever?
A body temperature of 99.2°F (37.3ºC) could be a normal reading. The normal range is 97.5–99.5°F (36.4–37.5ºC). To some extent, it depends on what is normal for the person. In a child, an oral temperature over 99°F (37.2ºC) indicates a fever.
Is 99°F (37ºC) a fever under the tongue?
According to the American Academy of Pediatrics, an oral reading above 99°F (37.2ºC) is a true fever. In an adult, however, this reading is not a fever. It falls within the normal range of 97.5–99.5°F (36.4–37.5ºC).
A low grade fever for a baby is a temperature at or above 100.4°F (38ºC) rectally or 99.5°F (37.5ºC) orally. In an adult, a low grade fever is generally an oral temperature of 100.4°F (38ºC) to 104°F (40ºC).
A low grade fever is not always a cause for concern.
In both children and adults, the most common cause of a low grade fever is a viral infection. The fever will get better as your body fights off the infection. In babies younger than 2 months old, a fever is a reason to contact a pediatrician right away.