For centuries people have debated whether fever is helpful or harmful. Hippocrates thought fever could “cook” illness out of a patient, while later physicians regarded fever as a dangerous disease to be treated. Today, researchers recognize fever as a conserved part of the immune response to pathogens, shared across many animal species. But exactly how elevated body temperature helps fight infections remains unclear.
Microbiologist Sam Wilson of the University of Cambridge says there are two main ideas. One is that the heat of a fever directly harms pathogens, similar to Hippocrates’ notion. The other is that fever helps by enhancing immune function, or that the temperature rise is an unavoidable side effect of immune activation. Because these effects usually occur together, separating them experimentally is difficult.
Wilson and colleagues addressed the question in a study published in Science. They chose avian influenza (bird flu) for the experiment because bird influenza A viruses replicate in birds at temperatures higher than those of human airways. Bird flus are adapted to the warmer gut environment of birds, roughly equivalent to human fever temperatures. The team identified a genomic segment, PB1, that helps bird flu replicate at higher temperatures and inserted it into a human influenza virus, creating two nearly identical viruses: a normal human strain and a heat-tolerant version.
Laboratory mice provided a useful model because, unlike many species, mice do not mount a fever in response to influenza infection. That allowed the researchers to simulate fever by housing some mice at slightly elevated ambient temperatures, then infect them with either the normal or heat-tolerant virus. At standard laboratory temperatures both groups of mice developed illness. When the environment was warmed, mice infected with the normal (non–heat-tolerant) strain were largely spared, while those infected with the heat-tolerant strain still got sick. The result suggests that elevated temperature alone can reduce replication and disease for temperature-sensitive viruses like human influenza.
Researchers not involved in the study praised the work but urged caution in generalizing the findings to humans. Daniel Barreda, a microbiologist at the University of Alberta, said the study supports the idea that temperature is an effective part of the body’s response, but it doesn’t exclude important immune-boosting effects of fever that might matter for other viruses. Emergency physician and researcher Joe Alcock noted that the study adds to evidence that fever evolved for a reason, and it should prompt reflection about routine fever suppression in clinical practice.
Treating fever with antipyretics such as acetaminophen or ibuprofen is common and often appropriate, since high temperatures can damage human tissues. But the study raises the still-unanswered question of whether reducing fever during a viral infection might sometimes impede the body’s ability to clear the virus. More research is needed to determine how fever, immune responses, and antipyretic use interact in humans and across different pathogens.