A fever that appears to rise and fall, often returning to a normal temperature before spiking again, is known as an intermittent fever. This pattern describes a temperature elevation that fully resolves to a baseline of 98.6°F (37°C) or lower at least once over a 24-hour period before the temperature climbs back up. This fluctuating behavior is a common manifestation that reflects the body’s dynamic battle against an underlying process, most often an infection. Understanding this cycle requires looking closely at how the body manages its temperature control.
Understanding the Body’s Temperature Cycle
The sensation of a fever that comes and goes originates in the hypothalamus, which functions as the body’s central thermostat. When the immune system detects an invading threat, it releases chemical messengers called pyrogens. These pyrogens travel to the hypothalamus, where they trigger the production of prostaglandin E2 (PGE2).
PGE2 acts to “reset” the hypothalamic temperature set point to a higher level. The body responds to this new, higher set point by initiating heat-generating actions, such as shivering and muscle contractions, to raise the core temperature. Once the body reaches this elevated temperature, the fever is established.
The intermittent pattern is often a reflection of the immune system’s fluctuating activity or the effect of medication. As the immune cells are released in bursts, the concentration of pyrogens in the bloodstream naturally waxes and wanes, causing the set point to briefly dip before the next wave of immune signaling. Furthermore, antipyretic medications like acetaminophen or ibuprofen inhibit the synthesis of PGE2. Once the drug’s effect wears off, the pyrogens still circulate, causing the hypothalamic set point to jump back up, resulting in a recurring fever spike.
Acute Infections Causing Intermittent Fevers
The most frequent causes of a fluctuating fever are acute infections, where the immune system is actively fighting off a pathogen. Both viral and bacterial invaders can create this on-again, off-again temperature pattern as the body cycles through periods of intense defense and temporary lulls. Many common viral illnesses, including influenza or COVID-19, often present with fevers that spike in the late afternoon or evening.
This timing may be linked to the body’s natural circadian rhythms, which can influence immune system activity and the release of temperature-regulating hormones. For bacterial infections, such as a urinary tract infection (UTI), pneumonia, or an abscess, the intermittent pattern can be a sign of the immune system attempting to contain the infection or a fluctuating bacterial load. The fever may temporarily drop after a burst of immune activity or when a localized collection of pus releases its contents into the bloodstream.
Certain infections are known for producing predictable, cyclical fevers. For instance, some forms of malaria, caused by a parasite, result in fever spikes that occur every 48 or 72 hours, depending on the species involved. This timing corresponds to the parasite’s life cycle, specifically when it bursts out of red blood cells and triggers a large immune response. The fever subsides once the parasites re-enter the blood cells and the cycle repeats.
Non-Infectious Conditions and Drug Reactions
While infection is the most common cause, a fluctuating fever can also signal a non-infectious inflammatory process or a reaction to medication. Autoimmune disorders are a significant group of non-infectious conditions that can cause recurring fevers. Diseases like systemic lupus erythematosus (lupus) or rheumatoid arthritis involve the immune system mistakenly attacking the body’s own healthy tissues.
These conditions often cause fevers during flare-ups, where systemic inflammation is high, leading to a release of inflammatory cytokines that act as pyrogens. A specific type of inflammatory disorder, Adult-onset Still’s disease, is particularly characterized by a quotidian fever, which is a spike that reliably occurs once or twice a day, often in the late afternoon or evening.
Certain medications can induce a drug-induced fever, where the body’s thermoregulation is disrupted as a side effect. Antibiotics, anticonvulsants, and certain heart medications are known examples that cause a fever that resolves once the medication is stopped. Less commonly, a fluctuating fever can be the first sign of a malignancy, such as lymphoma, where cancer cells release pyrogens that cause the fever. Finally, genetic conditions known as periodic fever syndromes, such as Familial Mediterranean Fever (FMF), cause episodes of fever and inflammation that recur at predictable intervals without any infection.
When Fluctuating Fevers Require Medical Evaluation
While most intermittent fevers are part of the body’s normal response to a self-limiting illness, certain signs should prompt a medical assessment. An adult fever that reaches or exceeds 103°F (39.4°C) warrants attention, especially if it does not respond to fever-reducing medication.
The duration of the fever is also an indicator; a fluctuating temperature that persists for more than 48 to 72 hours without clear improvement should be evaluated. Fevers that reoccur regularly over a longer period, such as a week or more, may signal a more complex underlying condition.
Immediate medical attention is necessary if the fluctuating fever is accompanied by severe symptoms that suggest a serious issue. These associated “red flags” include:
- A severe headache or a stiff neck.
- Mental status changes like confusion or extreme sleepiness.
- Difficulty breathing.
- The presence of a new rash, particularly one that looks like small bleeding spots.
- Unexplained severe pain in the abdomen or chest.