A fever is an elevation of the body’s temperature above the normal range of approximately 97°F to 99°F (36.1°C to 37.2°C). This rise in temperature is often associated with the immune system actively fighting off an infection. However, many people report that they rarely, if ever, develop a fever when sick, prompting questions about their body’s defense mechanisms. Understanding this absent response requires examining the physiological process that creates a fever and the factors that can mask or inhibit it.
The Biological Purpose of Fever
Fever initiation begins when the body encounters foreign invaders, such as bacteria or viruses, or experiences inflammation. These threats release pyrogens, which are substances that can be exogenous (from the pathogen) or endogenous (released by the body’s immune cells). Immune cells, such as macrophages, respond by releasing endogenous pyrogens like Interleukin-1 (IL-1) and Tumor Necrosis Factor-alpha (TNF- \(\alpha\)).
These chemical messengers travel to the hypothalamus, the brain region that acts as the body’s central thermostat. There, they trigger the production of Prostaglandin E2 (PGE2). PGE2 effectively “resets” the hypothalamic temperature set point to a higher level, signaling the body to conserve heat and increase heat production through actions like shivering and vasoconstriction.
The higher temperature inhibits the replication and growth of many pathogens, creating an unfavorable environment. Simultaneously, the increased heat enhances the function of various immune cells, including increasing the activity of phagocytes and promoting the proliferation of T-cells. This coordinated response suggests that a fever is a highly regulated defense mechanism.
Common Reasons for Suppressed or Absent Fever
The most frequent reason for an absent fever relates to the common use of over-the-counter pain relievers and cold medications. Drugs like acetaminophen and non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and naproxen inhibit the synthesis of PGE2 in the hypothalamus. By blocking this molecule, these medications prevent the hypothalamus from resetting the body’s thermostat, suppressing the fever response.
Natural variation in individual body temperature is another significant factor. A person’s baseline temperature can fluctuate throughout the day, and some individuals have a lower average resting temperature than the often-cited 98.6°F (37°C). For these people, a clinically significant rise in temperature might still fall below the traditional fever threshold of 100.4°F (38°C).
Age plays a role in the blunted fever response, particularly in older adults. The aging immune system is less effective at mobilizing the immune cells needed to release the pyrogens that initiate temperature elevation. Studies indicate that fever is absent or diminished in up to 20–30% of older adults with confirmed infections. Therefore, a smaller temperature deviation from their personal baseline is a more important indicator of illness.
Measurement technique can also introduce error, leading to an inaccurate reading. The temperature taken with a temporal scanner on the forehead or an oral thermometer can be significantly lower than a core body temperature measurement, such as a rectal reading. An individual may technically be running a fever, but the chosen method of measurement fails to capture the true elevation.
Medical Conditions Associated with Low Fever Response
The absence of a fever may point toward a complex underlying medical reason related to a compromised immune system. Individuals undergoing treatments that intentionally dampen the immune response, such as chemotherapy or long-term corticosteroid use, often lack the ability to mount a febrile response. These treatments interfere with the production and function of the immune cells necessary to release the endogenous pyrogens.
Chronic health conditions that affect the immune system can also lead to an absent or diminished fever. Patients with primary immunodeficiency (PI) disorders frequently report a lack of fever during an infection. Similarly, conditions like advanced HIV/AIDS destroy immune cells and can prevent the necessary inflammatory cascade from occurring, masking a serious underlying infection.
In extremely severe infections, such as sepsis, a failure to mount a fever response can be an ominous sign rather than a benign one. Instead of a fever, the body may exhibit hypothermia, which is an abnormally low body temperature. This indicates the immune system is overwhelmed and failing to regulate the core temperature. While rare, conditions affecting the hypothalamus itself, such as brain trauma or certain neurological disorders, can directly impair the body’s temperature regulation center, preventing the fever mechanism from activating.