Spiking A Fever Can Stop Seasonal Flu Cold, But Bird Flu Doesn’t Break A Sweat

When seasonal influenza strikes, the body cranks up its internal thermostat as a first line of defense. That miserable fever might feel like a bug, but it’s actually a a shield of sorts that helps restrict how much the virus can spread through your respiratory system. Research reveals just how powerful this temperature defense really is. In mouse experiments, raising body temperature by about 2°C turned a typically severe infection into a mild one.

But there’s a catch: bird flu doesn’t play by the same rules.

Scientists at the University of Cambridge and collaborating institutions discovered that avian influenza viruses carry a genetic advantage allowing them to shrug off febrile temperatures that would cripple their seasonal cousins. The finding, published in Science, helps explain why bird flu and pandemic flu strains cause such devastating disease in humans, and it raises questions about the widespread use of fever-reducing medications during flu season.

In experiments with mice, researchers found that changing just two amino acids in a viral protein was enough to determine whether a fever would protect against severe disease or fail completely.

How Seasonal Flu Gets Stopped Cold

Human seasonal flu viruses adapted to thrive in the upper respiratory tract, where temperatures hover around 33°C (91°F). When the body raises its temperature to 39-40°C during a fever, these viruses struggle to replicate efficiently. The research team demonstrated this by infecting mice with a laboratory flu strain at normal room temperature versus an elevated temperature that simulated fever.

Mice kept at standard room temperature after infection experienced severe weight loss and many had to be euthanized. Meanwhile, mice housed in warmer conditions that raised their core body temperature by about 2°C maintained healthy weights throughout the infection. The elevated temperature reduced virus levels in the lungs by nearly tenfold within 24 hours.

This protective effect wasn’t due to a ramped-up immune response. Researchers confirmed that the higher temperature directly inhibited viral replication, independent of inflammation or other immune processes.

Temperature Creates a Species Barrier

Birds carry influenza in their gastrointestinal tracts at body temperatures of 40-42°C, which is the same temperature range as a human fever. When the research team tested various avian flu strains in human lung cells, they found these viruses replicated efficiently at 40°C, unlike human seasonal strains that were heavily restricted.

The key difference lies in a viral component called PB1, part of the machinery flu uses to copy itself. The team created chimeric viruses — essentially taking human flu and swapping in components from avian strains to identify which parts conferred temperature resistance. PB1 emerged as the dominant factor.

Historical pandemic flu viruses from 1918, 1957, and 1968 all acquired avian-origin PB1 proteins through genetic reassortment. Those pandemics killed millions more people than typical seasonal flu outbreaks, which claim an estimated 290,000 to 650,000 lives annually worldwide.

A Two-Degree Difference Between Life and Death

To test whether temperature resistance actually translated to more severe disease, researchers engineered a virus that was identical to the human seasonal strain except for two amino acid changes in PB1. These mutations gave the virus the same temperature resistance seen in avian strains.

Both the original and modified viruses caused severe illness in mice kept at normal temperatures. But when the ambient temperature was raised to simulate fever, outcomes diverged dramatically. The original virus caused only mild disease, with no mice requiring euthanasia. The temperature-resistant mutant, however, caused severe illness and death even in the presence of elevated body temperature.

The approximately 2°C temperature increase, equivalent to a typical human fever, made the difference between survival and severe disease.

Should You Skip the Tylenol?

Fever is routinely treated with medications like acetaminophen and ibuprofen, both at home and in hospitals. The new findings raise an intriguing question: if elevated temperature itself slows seasonal flu replication, could lowering a fever blunt that natural defense? The study didn’t test fever-reducing drugs, so researchers can’t say what this means for real-world treatment.

Clinical evidence hints this question is worth exploring. Flu patients who don’t develop fever despite being symptomatic often have higher mortality rates and longer hospital stays compared to patients who do run fevers. Animal studies have shown that suppressing fever can enhance viral replication.

The researchers stop short of making clinical recommendations, noting that human trials are needed. For now, decisions about fever medicine still belong with you and your clinician, not in a mouse study. But the findings add important context to those conversations.

Bird Flu Surveillance Gets a New Tool

The research has direct relevance for monitoring emerging flu threats. Highly pathogenic H5N1 strains have caused sporadic human infections over the past decades, with some lineages showing mortality rates of 50-60%. Temperature resistance in the viral polymerase could become one more clue scientists watch when they assess pandemic potential.

If a virus carries the genetic signatures that enable replication at febrile temperatures, it may be more likely to cause severe disease if it adapts to spread efficiently between humans. Past pandemic strains eventually evolved to become the seasonal flu we deal with today, losing their temperature resistance over decades of circulation in human populations.

The two-degree difference between bird and human body temperatures remains a critical dividing line. Fever is a more powerful antiviral defense than previously recognized, powerful enough to turn severe infections into mild ones in this mouse model. But only if the virus can feel the heat.

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Journal: Science, Volume 390, November 27, 2025 | Title: Avian-origin influenza A viruses tolerate elevated pyrexic temperatures in mammals | DOI: 10.1126/science.adq4691

Affiliations: University of Glasgow Centre for Virus Research, Cambridge Institute of Therapeutic Immunology and Infectious Disease, University of Oxford, National Institutes of Health, University of Edinburgh, University of Tokyo, US National Poultry Research Center, Walter and Eliza Hall Institute of Medical Research, University of Melbourne