Until a few decades ago, fevers were thought to be an unavoidable but unpleasant side effect of illness. But modern research suggests that most fevers are actually integral to effective immune responses.

The evolutionary importance of the fever was first examined by comparing fevers to other dangerous spikes in temperature. Writing in American Zoologist, physiologist Matthew Kluger notes that responses to increased body temperature are different when the cause is a fever compared to, say, exercising in the heat. When our body temperature rises in response to an infection, we take actions consistent with trying to warm up even more: shivering, huddling under blankets. When hyperthermic, we sweat, feel the urge to drink cool water, and make every effort to cool down. In other words, when we’re sick, our bodies try their best to be hot.

Kluger also points out that fevers occur in a wide variety of animals besides mammals. Ectothermic animals such as fish and lizards do not internally control their own body temperature. They must rely on their environment to warm up or cool down. When infected, some lizards will crawl out onto hot rocks to warm up more; fish might swim to warmer water. Such a broad presence across multiple phyla suggests a basic evolutionary origin for fevers.

So how do fevers help? Kluger suggests that the elevated temperature may directly harm certain pathogens, and, additionally, elevated temperatures may improve certain immune responses. Infectious disease expert Norbert Roberts, Jr., expands on this idea in Reviews of Infectious Diseases, noting that fevers aid a few different aspects of a healthy immune response. Increased temperature especially seems to help early on, when certain white blood cells are ramping up and recognizing a pathogen to attack. Some of these responses are actually suppressed at lower temperatures, so not only can a fever be good, but a lack of fever can be bad. Once the immune response reaches the effector stage, where cells are actively attacking a pathogen, elevated temperature is no longer needed.

There are downsides: the effort required to raise body temperature increases stress and puts a strain on the heart, so fevers can be dangerous for those with heart disease or the elderly. If body temperature rises too high, Roberts notes that rather than benefitting, the immune response may be harmed. There is a febrile goldilocks zone of elevated, but not excessive, temperature increase.

These findings call into question whether reducing fevers is worth it. Philip Mackiowac, of the Veteran Affairs Maryland Healthcare System, believes the answer is typically no, for all the aforementioned reasons. He points to studies showing that infections take longer to ebb in patients when fevers are actively suppressed. In general, if there are no underlying health conditions, he suggests fevers should be allowed to run their course, if they aren’t too high and don’t last too long. Sometimes feeling lousy is a good thing.