Fever Isn’t Enough: How Bird Flu Outsmarts One of Humanity’s Oldest Defenses
New evidence is shaking up what scientists thought they knew about bird flu — and it could have serious consequences for humans. According to groundbreaking research led by the Universities of Cambridge and Glasgow, bird flu viruses can continue to multiply even at temperatures that typically shut down other viruses during a fever. And this is the part most people miss: our body’s natural heat defense may not be enough to stop them.
A Closer Look at Why This Matters
Fevers are one of the body’s go-to defenses. When a virus attacks, body temperature rises — often reaching up to 41°C — in an effort to create a hostile environment for viral growth. For most human flu viruses, that’s enough to weaken their replication. But the new study, published in Science, has uncovered why avian influenza behaves differently.
Researchers pinpointed a specific viral gene that determines how sensitive a flu strain is to temperature. Historically, this same gene has been linked to past deadly pandemics — including those in 1957 and 1968 — when it jumped from bird flu strains to human flu viruses, making them more adaptable and virulent.
Human vs. Bird Flu: A Tale of Two Environments
Human seasonal flu viruses (influenza A) prefer cooler conditions around 33°C, typically found in the upper respiratory tract — the nose and throat area. In contrast, bird flu viruses thrive in much warmer regions, like the lower lungs or even the guts of birds such as ducks and seagulls, where temperatures naturally hover around 40–42°C.
That temperature advantage means avian viruses are used to surviving, even under the intense heat of fever. And here’s where it gets controversial: while fever can act as a powerful antiviral barrier against human-origin flu, the same mechanism doesn’t necessarily work on avian strains.
Simulating Fever in the Lab
Using mice, the research team recreated fever-like conditions to test how influenza viruses respond to elevated temperatures. Mice don’t typically develop fevers during flu infections, so researchers simulated the effect by raising the environmental temperature around them. When the mice were exposed to a human-adapted flu virus, known as PR8, higher body temperatures effectively suppressed viral replication — even turning severe infections into mild ones with just a two-degree increase.
However, when infected with avian-like strains, the same temperature elevation made little difference. The viruses withstood the heat, multiplying rapidly and causing severe disease. The crucial factor behind this resilience turned out to be the PB1 gene — a component that helps the virus replicate its genetic material inside infected cells.
Why This Genetic Trick Is So Dangerous
Here’s the twist: avian and human flu viruses can exchange genes when they infect the same host, such as pigs. This process, known as reassortment, can create entirely new strains with unpredictable properties. Dr. Matt Turnbull from the University of Glasgow warns that these genetic swaps have been the root cause of past pandemics: “In 1957 and 1968, human flu viruses swapped their PB1 gene with avian viruses — and that may explain why those outbreaks were so severe.”
So, what does this mean today? Monitoring bird flu strains for fever resistance could be a crucial part of spotting potential pandemic threats early. If scientists can identify which strains tolerate high temperatures, they might be able to predict which ones pose the greatest danger to humans.
Deadly Potential of Bird Flu in Humans
Professor Sam Wilson from the University of Cambridge points out that while human infections from bird flu remain rare, the outcomes are often grim. Mortality rates during outbreaks of H5N1 bird flu, for example, have exceeded 40%. “Understanding why these viruses cause such severe illness in people,” Wilson explains, “is key to improving surveillance and readiness for future pandemics.”
Rethinking Fever Treatment
The study also raises important questions about how fevers are treated. Common medications such as aspirin and ibuprofen lower body temperature, which might provide comfort but could also inadvertently help the virus. Clinical studies have suggested that suppressing fever in influenza patients can sometimes lead to greater viral spread. Could treating a fever actually make the illness worse? It’s a question researchers are now revisiting with renewed urgency.
A Shift in How We View Cross-Species Infection
Professor Wendy Barclay of the UK Medical Research Council’s Infections and Immunity Board describes the study as an "elegant" demonstration of how animal body temperatures influence viral behavior. Different species host viruses in unique thermal environments. When viruses leap from one species to another — say, from bird to human — their ability to cope with fever could determine how dangerous they become.
Barclay emphasizes that the temperature resilience conferred by the PB1 gene explains why bird-origin viruses can sometimes resist the fever response that typically limits human flu. These findings highlight a pressing need to rethink when and how to use fever-reducing drugs during influenza infections — and could even reshape strategies for pandemic preparedness.
Funding and Credits
This international research effort was supported primarily by the UK Medical Research Council, along with contributions from the Wellcome Trust, Biotechnology and Biological Sciences Research Council, European Research Council, EU Horizon 2020, the UK Department for Environment, Food & Rural Affairs, and the U.S. Department of Agriculture.
Reference: Turnbull, ML et al. Avian-origin influenza A viruses tolerate elevated pyrexic temperatures in mammals. Science, 27 Nov 2025. DOI: 10.1126/science.adq4691.
Bird flu’s ability to survive fever could change how we understand — and fight — future pandemics. But it also raises a critical question: should we still be treating fevers so aggressively, or are we undermining one of our own natural defenses? What do you think — should fever suppression be reconsidered during viral outbreaks?
