What Is a Low-Grade Fever?

Most healthcare providers define a low-grade fever as a temperature between 99 degrees F (37.2 C) and 100.4 degrees F (38.0 C). A normal body temperature is typically described as 98.6 F (37 C) but can range from 97 F (36.1 C) to 99 F (37.2 C).

A low-grade fever is often the first sign of an infection. When the body temperature is normal, infectious bacteria and viruses are able to thrive. A fever is the body's way of creating a hostile environment to help neutralize them.

Low-grade fevers also have non-infectious causes, including diseases, vaccines, and medications that affect the body's "thermostat," known as the hypothalamus.

This article explains the symptoms and causes of low-grade fevers. It also offers tips on how to manage a fever and when it is time to see a healthcare provider.

Low-Grade Fever Symptoms

Although fever is the body's defense against infection—increasing the internal temperature to where most bacteria and viruses are unable to survive—it can cause a cascade of symptoms the higher the temperature gets.

With a low-grade fever, the condition is often asymptomatic (without symptoms) or subclinical (not severe enough to cause definite or recognized symptoms). If symptoms do occur, they will often be mild and cause:

• Headache
• Fatigue and tiredness
• General achiness
• Mild sweating

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

When to See a Healthcare Provider

There are times when a fever warrants a call to your healthcare provider or an immediate visit to the emergency room.

Adults

Seek medical care for a fever that appears with these other symptoms:

• Stiff neck, which can indicate a potentially life-threatening condition called meningitis affecting the brain and spinal cord
• Feeling sleepy, dull, or confused
• Persistent vomiting and/or diarrhea
• Persistent sore throat
• Seizure or loss of consciousness
• Pain or burning with urination
• A rash that looks like purple spots

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

Infants and Children

Seek medical help right away if you child:

• Seems lethargic, uncharacteristically fussy, or seems very ill
• Has severe symptoms such as a stiff neck, a severe headache, or severe sore throat
• Seems to be having trouble breathing
• Has a rash accompanying the fever
• Has repeated vomiting or diarrhea with the fever
• Has underlying medical issues such as a heart condition or cancer
• Takes steroids or immune-suppressant medication
• Has a seizure (rapidly rising fevers can sometimes trigger febrile seizures in young children)
• Is under 3 months and has a fever of 100.4 or higher
• Has a fever of 104 or higher (at any age)

Call your child's healthcare provider if:

• Your child is 2 years old or younger and their fever doesn't go away after 24 hours.
• Your child is older than 2 and their fever doesn't go away after 72 hours.
• Their condition seems to be worsening.
• They act sick after you've given them a fever reducer.
  
• There are any other symptoms that worry you. Follow your instincts and seek medical care.

What Causes a Low-Grade Fever?

A fever—even a low-grade one—is a sign that something is wrong in your body. It may indicate that your immune system is mounting a defense against a foreign invader like a virus or bacteria. Or, it may be due to non-infectious causes such as autoimmune diseases, medications, or even blood cancer.

Whatever the underlying cause, fever is ultimately the result of a trigger known as a pyrogen. This is any substance from within or outside of the body that stimulates the production of a substance called prostaglandin E2 (PGE2).

PGE2 acts on a part of the brain, called the hypothalamus, which regulates body temperature. It signals the hypothalamus to increase the temperature set point, causing the body to heat up.

There are both exogenous pyrogens like viruses, bacteria, parasites, and fungi that come from outside of the body and endogenous pyrogens like interferons and interleukins that are released by the body in response to injury or disease.

Infectious Causes

Viral, bacterial, parasitic, and fungal infections can all cause low-grade fevers. Some function independently as pyrogens (like viruses), while others release toxins that are pyrogenic (like bacteria).

Examples include:

• Chickenpox
• Cholera
• Coccidioidomycosis (Valley fever)
• Cold viruses (including rhinoviruses)
• COVID-19
• Infectious mononucleosis
• Influenza (flu)
• Malaria
• Measles
• Mumps
• Otitis media (middle ear infection)
• Pneumonia
  
• Rubella (German measles)
• Sinusitis (sinus infections)
• Strep throat
  
• Urinary tract infections
  
• Viral and bacterial gastroenteritis (stomach flu)
  

Non-Infectious Illnesses

There are diseases and conditions that can directly or indirectly trigger a low-grade fever. These include autoimmune diseases, hormonal disorders, inflammatory diseases, and conditions that cause the obstruction of major blood vessels or airways.

Examples include:

• Acute respiratory distress syndrome (ARDS)
• Adrenal insufficiency (Addison's disease)
• Aspiration pneumonia
• Behçet syndrome
• Blood transfusion
• Cholecystitis
• Deep vein thrombosis (DVT)
• Gout
• Hyperthyroidism
• Menstruation
• Leukemia
• Lupus
• Lymphoma
• Pancreatitis
• Pulmonary embolism
• Stress (psychogenic fever)
• Stroke
• Rheumatoid arthritis

Certain medications can also trigger a low-grade fever, including:

• Anticonvulsant drugs like phenytoin and carbamazepine
• Antipsychotic drugs like haloperidol
• Beta-lactam antibiotics
• Cardiac arrhythmia drugs like amiodarone
• HIV drugs like abacavir
• Selective serotonin reuptake inhibitors (SSRI) antidepressants
• Sulfa drugs
• Vaccines
  

How to Treat a Low-Grade Fever

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

If you decide to treat a low-grade fever, over-the-counter painkillers like Tylenol (acetaminophen) and Advil (ibuprofen) 410 are good choices.

Aspirin is another option, but never give aspirin to children unless instructed by your healthcare provider. Aspirin is associated with a rare but potentially life-threatening reaction called Reye's syndrome affecting children and teens with a virus-induced fever.

Other self-help measures include:

• Drinking plenty of fluids to prevent dehydration caused by sweating and fever
• Resting to help your body heal itself
• Wearing light, breathable fabrics
• Placing a cool, moist cloth on your forehead
• Taking a cool bath (but not an ice bath which can cause hypothermia and increase your body temperature)

Summary

A low-grade fever is between 99 degrees F (37.2 C) and 100.4 degrees F (38.0 C). It is usually nothing to worry about and will typically run its course in several days or less without treatment. Infections are the most common cause, but there are also non-infectious causes (including medications and stress).

Seek immediate medical care for a low-grade fever in children 3 months or under and anyone whose fever lasts for more than 72 hours.

A Word From Verywell

A low-grade fever, while not as dangerous as a high-grade fever, comes with its own challenges. Keep things like understanding body temperature, temperature measurement methods, nutrition, and alcohol consumption in mind to ensure you or your loved ones stay safe and healthy.

— KASHIF J. PIRACHA, MD, MEDICAL EXPERT BOARD