Instalab ResearchIn healthy adults, a normal resting heart rate typically ranges from about 60 to 100 beats per minute, though many cardiologists consider the lower half of that range to be optimal. Fit individuals, such as endurance athletes, often have resting heart rates in the 40s or 50s. Women tend to have slightly higher rates than men. Large-scale studies of healthy populations confirm this range, with average resting rates of about 60 to 64 in women and 58 to 62 in men. Anything consistently over 100 at rest is considered tachycardia, a medical term for abnormally fast heart rhythm.
When the body develops an infection, one of its natural defense mechanisms is fever. Raising body temperature helps slow microbial replication and boosts immune efficiency. But fever also activates the autonomic nervous system, which increases heart rate. This phenomenon is well-documented: for every 1°C rise in body temperature, heart rate tends to increase by about 8 to 10 beats per minute. That means if your baseline heart rate is 70, and you spike a fever of 39.5°C (103.1°F), your heart rate could reasonably rise to near 100. For some individuals, the increase is even steeper.
This fever-related tachycardia is so common that clinicians use it as a diagnostic clue when evaluating infection severity. In pediatric cases, for example, resting heart rate has been shown to strongly correlate with disease severity, with values above 96 beats per minute predicting more severe infection.
A heart rate of 120 at rest is significantly above the normal adult range and falls squarely into tachycardia territory. The key is understanding the context. If you are lying in bed, feeling weak, and your temperature is 39 to 40°C, a pulse of 120 may reflect your body’s natural response. However, such a high resting rate should not be ignored. Elevated resting heart rates during illness have been associated with greater risk of complications, particularly in children and those with pre-existing heart conditions.
Additionally, not all illnesses cause the same heart rate changes. Some infections are notorious for triggering unusually high heart rates, while others may cause relative bradycardia, a slower-than-expected pulse for the degree of fever. This has been observed in infections such as scrub typhus and typhoid fever. In other words, while 120 is not unheard of, it is still an outlier for most healthy adults and could indicate either a severe infection or an underlying cardiovascular strain.
Persistent tachycardia is not just uncomfortable, it can strain the cardiovascular system. Studies have consistently found that elevated resting heart rates are linked with higher long-term risk of cardiovascular disease and mortality. In the context of acute illness, sustained tachycardia can reduce heart efficiency, increase oxygen demand, and worsen fatigue. In some cases, it may even lead to rhythm disturbances.
Infections that inflame the heart muscle, such as viral myocarditis or endocarditis, can directly disrupt electrical conduction, compounding the risk. For this reason, clinicians take note when a patient’s heart rate remains elevated despite fever control, as this can signal complications.
If you are sick and notice your resting heart rate is consistently around 120 or higher, it is worth contacting a healthcare provider. This is especially urgent if the elevated heart rate is accompanied by chest pain, dizziness, shortness of breath, or low blood pressure. Even in the absence of those symptoms, tachycardia beyond 100 to 110 at rest should raise a flag. While fever may explain much of the increase, it is important to rule out dehydration, sepsis, anemia, thyroid abnormalities, or heart involvement.
