People have argued for centuries about whether fever helps or harms the sick. Hippocrates believed fever could “cook” illness out of a patient, while later doctors treated fever itself as a dangerous condition. Modern biology views fever as a conserved element of the immune response across many animals, but exactly how a raised body temperature helps suppress infections has remained unclear.
Microbiologist Sam Wilson of the University of Cambridge summarizes two main explanations. One is a direct effect: higher temperature harms pathogens, slowing their replication or survival. The other is an indirect effect: fever amplifies immune processes, or the temperature rise is a byproduct of immune signaling. Because heat and immune activation usually occur together, teasing apart their separate contributions is experimentally difficult.
Wilson and colleagues tackled that problem in a study published in Science using influenza viruses and mice. They started with a biological insight: avian influenza A viruses are adapted to replicate at the warmer temperatures of bird guts—temperatures similar to human fever—whereas human influenza strains are typically temperature-sensitive. The researchers identified a viral genomic segment called PB1 that enables bird flu to tolerate higher temperatures and inserted it into a human influenza strain, producing two nearly identical viruses: a standard human strain and a heat-tolerant version.
Mice offered a key experimental advantage: unlike many species, mice do not consistently mount a fever in response to influenza infection. That allowed the team to simulate fever simply by raising the animals’ ambient temperature. At normal lab temperatures both groups of mice became ill when infected. When housed at slightly higher temperatures, however, mice infected with the ordinary, temperature-sensitive human strain were largely protected, while mice infected with the PB1-modified, heat-tolerant strain still fell ill. These results indicate that elevated temperature alone can reduce replication and disease for temperature-sensitive viruses such as typical human influenza.
Outside researchers praised the study as thoughtful and informative but urged caution in extrapolating to humans and all pathogens. Daniel Barreda, a microbiologist at the University of Alberta, said the work supports temperature as an effective component of host defense but does not rule out important immune-enhancing roles of fever for other infections. Emergency physician and researcher Joe Alcock suggested the findings add to the idea that fever evolved for a reason and should encourage reconsideration of routine fever suppression.
Treating fever with antipyretics like acetaminophen or ibuprofen is common and often appropriate, especially when high temperatures risk tissue damage. The new study, however, raises the still-open question of whether reducing fever during some viral infections might impede viral clearance. Further research is needed to map how temperature, immune responses, and antipyretic use interact in humans and across different pathogens.