Viral infections can lower body temperature below the normal range, and when they do, the drop often signals a more dangerous situation than a fever would. Most people associate infection with feeling hot, but a subset of patients with illnesses like COVID-19, influenza, and viral encephalitis develop hypothermia instead of, or alongside, fever. The phenomenon is well documented in clinical literature and carries real prognostic weight, yet it remains poorly understood by the general public.
How Viruses Can Push Temperature Down
Your body’s temperature is not a passive reading like a thermometer sitting in a room. It is actively regulated by a control center in the brain, and that control center responds to chemical signals released during infection. Among the most studied of those signals is tumor necrosis factor-alpha, or TNF-alpha, a molecule your immune system produces in large quantities when it detects a pathogen. TNF-alpha has a strange dual personality: depending on the nature of the inflammatory trigger, it can act as a fever-inducing signal or as a temperature-lowering one.
Research has shown that TNF-alpha can function as what immunologists call an “endogenous cryogen,” a substance produced by the body itself that drives temperature downward.1PubMed. Hypothermia in systemic inflammation: role of cytokines Whether it pushes temperature up or down depends on context, including the specific inflammatory stimulus and the balance of other immune signals present at the time.2Neuroimmunomodulation. Cytokines and Fever Another molecule, IL-10, appears to modulate TNF-alpha’s production, potentially influencing whether the net effect on temperature is heating or cooling.1PubMed. Hypothermia in systemic inflammation: role of cytokines
The upshot is that the immune response to a viral infection is not a simple thermostat turned to “high.” It is a complex negotiation among dozens of signaling molecules, and in some patients the result of that negotiation is a body temperature that falls rather than rises. When the immune system is overwhelmed, or when the body’s energy reserves are depleted, the thermostat can be reset downward rather than upward.
COVID-19 and Low Body Temperature
The COVID-19 pandemic produced an enormous amount of clinical data, and some of the clearest evidence that viral infections cause hypothermia came from hospitalized COVID patients. In a large study of more than 24,000 COVID-19 cases, patients whose initial body temperature was below 36°C (about 96.8°F) had roughly three times the odds of dying compared to those whose temperature fell in the normal 36 to 37°C range. Every single patient in that dataset who never managed to raise their temperature above 36°C died.3PubMed Central. Body temperature as a predictor of mortality in COVID-19
A separate study looking specifically at critically ill COVID patients found that those who were hypothermic on admission to the ICU were about twice as likely to die as non-hypothermic patients. They also spent longer on ventilators and had longer ICU stays overall.4PubMed Central. Hypothermia is Associated With Poor Prognosis in Hospitalized Patients With Severe COVID-19 Symptoms Case reports captured individual patients presenting to the hospital with confirmed COVID-19 and temperatures as low as 94.5°F, well below the threshold for clinical hypothermia, without any obvious external cause like cold exposure.5PubMed Central. Acute hypothermia in a patient with COVID-19: a case report and summary of the evidence
The pattern was not unique to COVID. A review of 10,000 non-COVID patients with sepsis, including cases of viral origin, found that the mortality rate among those who were hypothermic was around 47%.6PubMed Central. Association of hypothermia with increased mortality rate in SARS-CoV-2 infection Hypothermia during severe infection is not an oddity confined to one virus. It shows up across different pathogens and consistently predicts worse outcomes.
Why a Low Temperature During Infection Is a Red Flag
Fever gets all the attention because it is common, uncomfortable, and easy to detect. But in many respects, hypothermia during infection is a more worrying sign. Fever is generally an active, energy-intensive immune response: the body deliberately raises its temperature to create an environment less hospitable to pathogens and to speed up certain immune functions. Hypothermia, by contrast, often reflects the body’s inability to mount that response. It can indicate that the immune system is overwhelmed, that energy reserves are critically depleted, or that the infection has progressed to the point of circulatory collapse.
In the COVID-19 mortality data described above, the risk relationship between temperature and death was not a simple straight line. High fevers carried increased risk too, but the mortality jump at the low end of the temperature spectrum was stark. Among ventilated COVID patients who had been hypothermic, 75% died.6PubMed Central. Association of hypothermia with increased mortality rate in SARS-CoV-2 infection That number is grim enough to make the clinical point on its own: if you or someone you are caring for has a confirmed viral infection and a dropping temperature, that is not a reassuring sign. It warrants urgent medical attention.
One reason hypothermia gets overlooked is that patients and even some clinicians interpret a “normal” or low temperature as evidence that the infection is mild. In reality, the absence of fever does not mean the absence of severity. It can mean the opposite.
Why Older Adults and Newborns Are Especially Vulnerable
Two populations are particularly prone to developing hypothermia rather than fever during viral infections: older adults and newborns. In both groups, the body’s thermoregulatory system works differently than it does in a healthy younger adult.
In older adults, the fever response to infection is blunted or absent roughly 20 to 30 percent of the time.7Clinical Infectious Diseases. Fever in the elderly This means that a significant fraction of elderly patients with serious viral infections will never develop the cardinal sign that alerts most people to the presence of infection. They may instead present with confusion, fatigue, or a low temperature that gets chalked up to “just being cold.” This atypical presentation contributes to delayed diagnosis and treatment in a population already at elevated risk of complications.
In newborns, the picture is different but equally concerning. A systematic review of severe neonatal enterovirus infections found that temperature abnormalities, a category that includes both fever and hypothermia, were the most common symptom, appearing in more than half of cases.8PubMed Central. Clinical characteristics of severe neonatal enterovirus infection: a systematic review Newborns have a large surface area relative to their body mass, immature thermoregulatory centers, and limited metabolic reserves. A viral infection that would produce a robust fever in an older child can cause a temperature drop in a neonate. For parents, the practical takeaway is that a baby who feels cool to the touch and is feeding poorly deserves the same level of concern as one with a high fever.
When a Virus Attacks the Brain’s Thermostat Directly
Some viruses do not merely overwhelm the thermoregulatory system indirectly through immune signaling. They invade the brain regions responsible for temperature control. Japanese encephalitis virus, for example, can cross the blood-brain barrier and damage the thalamus and hypothalamus, structures that sit at the core of the body’s temperature-regulation circuit. A documented case involved an 18-year-old patient who developed intractable hypothermia after Japanese encephalitis virus infected her limbic system and caused hemorrhage in both thalami. Despite supportive care, the hypothermia proved uncontrollable and she died within two weeks.9PubMed Central. Hypothermia due to limbic system involvement and longitudinal myelitis in a case of Japanese encephalitis
This represents a fundamentally different mechanism from the cytokine-mediated temperature drop described earlier. In neuroinvasive infections, the virus physically damages the hardware rather than just disrupting the software. The hypothalamus normally integrates signals from the body and adjusts heat production and heat loss accordingly. If the virus destroys or inflames that tissue, the thermostat breaks entirely. Hypothermia from this cause tends to be more severe and harder to reverse than hypothermia driven by circulating immune signals.
Viruses known to be neuroinvasive, including certain strains of herpes simplex, West Nile, and Nipah virus, can all potentially cause this kind of thermoregulatory failure. Influenza viruses have also been shown to induce hypothermia. In mouse models, highly pathogenic H5N1 avian influenza viruses caused severe disease characterized by rapid weight loss, clinical deterioration, and induced hypothermia, with mortality rates between 75 and 100 percent.10PubMed Central. Distinct pathogenesis of hong kong-origin H5N1 viruses in mice compared to that of other highly pathogenic H5 avian influenza viruses While mouse physiology differs from human physiology in important ways, these findings demonstrate that viral infections can produce hypothermia through direct tissue damage and metabolic collapse, not just through subtle immune signaling.
Is Hypothermia During Infection a Survival Strategy?
There is an interesting evolutionary argument that hypothermia during infection is not simply a sign of failure. It may be, at least in some circumstances, a deliberate trade-off the body makes to survive. Fever is metabolically expensive. Raising body temperature by even one degree Celsius requires a substantial increase in energy expenditure. When the body is already depleted, sick, or malnourished, investing that energy into a fever response may not be the smartest bet.
One hypothesis proposes that fever and hypothermia represent two complementary survival strategies during severe infection. Fever is the active attack mode, creating conditions that hamper pathogen replication and enhance immune cell function. Hypothermia is the defensive retreat, conserving energy when reserves are critically low.11PubMed. Fever and hypothermia: two adaptive thermoregulatory responses to systemic inflammation Under this framework, the body shifts into hypothermia not because it has “given up” on fighting but because it has calculated, at some deep physiological level, that energy conservation is now more important than pathogen-killing.
Animal research lends some support to this idea. When infected rats were maintained under hypothermic conditions, they showed less organ dysfunction and less inflammation-related tissue damage, even though they had a higher burden of bacteria in their livers. The lower temperature appeared to dampen the inflammatory cascade enough to prevent the body from destroying itself in the process of fighting the infection.12Pathogens and Disease. Thermoregulation as a disease tolerance defense strategy In other words, hypothermia may sometimes function as a tolerance mechanism: the body accepts the presence of the pathogen in exchange for limiting the collateral damage caused by its own immune response.
This does not mean hypothermia during infection is harmless or that you should welcome it. In clinical settings, it consistently correlates with worse outcomes. But it does suggest that the relationship between body temperature and infection is more nuanced than the simple “fever fights germs” story most people carry around.
Getting an Accurate Reading
If you are monitoring your temperature during a viral illness, the method you use matters more than you might think. Different thermometers and different measurement sites produce readings that can vary by meaningful amounts. A study comparing various devices against a reference standard found that even among common consumer thermometers, mean differences ranged from less than a tenth of a degree Celsius for tympanic (ear) readings to over half a degree for thermal imaging cameras.13PubMed Central. Studying the Accuracy and Function of Different Thermometry Techniques for Measuring Body Temperature
A half-degree error in the wrong direction could be the difference between catching a low reading and missing it. If you suspect hypothermia, oral and tympanic thermometers are more reliable than forehead scanners or skin-surface devices. Taking multiple readings a few minutes apart, as clinicians did in the COVID-19 case report where the patient’s 94.5°F reading was confirmed over three consecutive measurements, is a sensible approach when the initial number seems unexpectedly low.5PubMed Central. Acute hypothermia in a patient with COVID-19: a case report and summary of the evidence
It is also worth knowing that “normal” body temperature is not a fixed 98.6°F (37°C) for everyone. Individual baselines vary, and they tend to run lower in older adults. If your baseline is already on the low side, a reading that looks technically normal could represent a meaningful drop for you. Tracking your temperature over several days during illness, rather than relying on a single snapshot, gives a much better picture of what is happening.
Temperature Problems After the Infection Clears
For some people, the disruption to body temperature regulation does not end when the virus is cleared. Post-viral autonomic dysfunction, a condition in which the nervous system that controls involuntary functions like heart rate, blood pressure, and temperature fails to return to normal after infection, has been documented in a number of post-COVID patients. A case series of seven patients with autonomic dysfunction following COVID-19 found that recurrent episodes of temperature instability, alongside tachycardia and blood pressure swings, persisted for months after the initial infection.14Annals of Medicine. A clinical case study of seven patients of autonomic dysfunction in post COVID-19 conditions with fever as the main clinical symptom
While that particular study focused mainly on recurrent fever episodes, the underlying mechanism, damage or dysregulation of the autonomic nervous system, can produce temperature swings in both directions. Some long COVID patients report unexplained low body temperatures alongside other autonomic symptoms like dizziness on standing, erratic heart rates, and abnormal sweating patterns. The thermoregulatory system, once disrupted by viral infection, does not always snap back neatly. For people experiencing ongoing temperature fluctuations after a viral illness, the issue is likely autonomic rather than a sign of persistent infection.
This is a relatively new area of study, and the evidence base is still thin. But the recognition that viruses can leave lasting marks on the body’s thermostat adds another dimension to the question posed in the title. Viral infections do not just temporarily push temperature down during the acute illness. In some cases, they alter the machinery of temperature control in ways that persist well beyond the infection itself.