Low-Grade Fever: what it usually means

Also searched as: mild fever, slightly elevated temperature, persistent low fever, subfebrile temperature · Updated 19 Aug 2026 · Educational — not medical advice

Illustrative — a virus particle, the usual reason for a mild fever

The short answer: A low-grade fever, typically between 99.5 and 100.2 degrees Fahrenheit (37.5 to 37.9 Celsius), is most often caused by a mild viral infection, recent vaccination, or ongoing inflammation somewhere in the body, and it usually resolves within a few days. Chronic infections and some autoimmune conditions can cause it to persist. A low-grade fever lasting longer than two weeks, or one accompanied by weight loss, night sweats, or fatigue, needs a medical work-up to identify the cause.

When to get medical help

A baby under 3 months with a temperature of 38°C (100.4°F) or higher — go to an emergency department now, unless a doctor can see your baby within 2 hours, even if the fever seems mild.

A low-grade fever persisting longer than two weeks needs medical evaluation to identify the cause.

Fever with unexplained weight loss, night sweats, or persistent fatigue should be evaluated promptly.

A low-grade fever with new joint pain, rash, or swollen glands warrants further work-up.

Fever that rises significantly or is accompanied by severe symptoms needs urgent medical attention.

Common causes

  • Mild viral infection — Common colds and other viral illnesses often produce a low-grade fever as the immune system responds.
  • Recent vaccination — The immune response to a vaccine can cause a brief, mild fever within a day or two of the shot.
  • Chronic or hidden infection — Ongoing infections such as urinary tract or dental infections can cause a persistent low-grade fever.
  • Autoimmune or inflammatory conditions — Conditions causing chronic inflammation can produce a low-grade fever without an infectious cause.
  • Medication reaction — Certain medications can trigger a mild drug fever as a side effect.
  • Thyroid or hormonal changes — An overactive thyroid can sometimes cause a mildly elevated baseline body temperature.

What helps

  • Stay well hydrated and rest while monitoring temperature over the following days.
  • Use over-the-counter fever reducers as directed if the fever causes discomfort.
  • Track the fever pattern, along with any other symptoms, to share with a doctor if it persists.
  • Avoid overdressing or overheating the environment while monitoring a mild fever.

Which doctor to consult

Family doctor

Start with a family doctor for evaluation and basic bloodwork; further specialist referral depends on what the initial work-up suggests.

Read more about the causes

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Lab tests doctors often check

If this symptom persists, these are the values a doctor commonly looks at — and if you already have a report, the analyzer explains where you stand.

WBC / Total Leukocyte CountCRPESRProcalcitonin

Questions people ask

What temperature counts as a low-grade fever?

A low-grade fever is generally considered a temperature between about 99.5 and 100.2 degrees Fahrenheit (37.5 to 37.9 Celsius), above the normal baseline but below a high fever. It often reflects a mild immune response and, when brief, does not usually require urgent treatment in older children and adults. A baby under 3 months is the exception: a temperature of 38°C (100.4°F) or higher needs a doctor straight away.

Why would a fever last for weeks without another obvious symptom?

A fever that persists for more than two weeks without a clear cause, sometimes called a fever of unknown origin, can result from a hidden infection, an autoimmune condition, or, less commonly, other chronic illnesses. This pattern warrants a structured medical work-up to identify the underlying cause.

Related symptoms

FatigueNight SweatsSwollen GlandsChills

Sources

This guide is educational and doesn't replace a clinical assessment. If a symptom is severe, rapidly worsening, or worrying you, see a doctor — being checked and reassured is never a wasted visit.