For years, scientists have warned that bird flu - better known as H5N1 - could one day make the dangerous leap from birds to humans and trigger a global health crisis.
Avian flu - a type of influenza - is entrenched across South and South-East Asia and has occasionally infected humans since emerging in China in the late 1990s. From 2003 to August 2025, the World Health Organization (WHO) has reported 990 human H5N1 cases across 25 countries, including 475 deaths - a 48% fatality rate.
In the US alone, the virus has struck more than 180 million birds, spread to over 1,000 dairy herds in 18 states, and infected at least 70 people - mostly farmworkers - causing several hospitalisations and one death. In January, three tigers and a leopard died at a wildlife rescue centre in India's Nagpur city from the virus that typically infects birds.
Symptoms in humans mimic a severe flu: high fever, cough, sore throat, muscle aches and, at times, conjunctivitis. Some people have no symptoms at all. The risk to humans remains low, but authorities are watching H5N1 closely for any shift that could make it spread more easily.
That concern is what prompted new peer-reviewed modelling by Indian researchers Philip Cherian and Gautam Menon of Ashoka University, which simulates how an H5N1 outbreak might unfold in humans and what early interventions could stop it before it spreads.
The model published in the BMC Public Health journal uses real-world data and computer simulations to play out how an outbreak might spread in real life.
The threat of an H5N1 pandemic in humans is a genuine one, but we can hope to forestall it through better surveillance and a more nimble public-health response, Prof Menon told the BBC.
A bird flu pandemic, researchers say, would begin quietly: a single infected bird passing the virus to a human - most likely a farmer, market worker or someone handling poultry. The danger lies not in that first infection but in what happens next: sustained human-to-human transmission.
Because real outbreaks start with limited, messy data, the researchers turned to BharatSim, an open-source simulation platform originally built for Covid-19 modelling, but versatile enough to study other diseases.
The key takeaway for policymakers is how narrow the window for action can be before an outbreak spirals out of control.
The paper estimates that once cases rise beyond roughly two to ten, the disease is likely to spread beyond primary and secondary contacts. Primary contacts are people who have had direct, close contact with an infected person, such as household members and caregivers, while secondary contacts are those who have had close contact with primary contacts.
If households of primary contacts are quarantined when just two cases are detected, the outbreak can almost certainly be contained. By the time 10 cases are identified, it is overwhelmingly likely that the infection has already spread into the wider population, making its trajectory virtually indistinguishable from a scenario with no early intervention.
The researchers chose a model of a single village in Tamil Nadu, which has a significant poultry industry. The simulations also highlighted an awkward trade-off: quarantining too early keeps families together for long stretches and increases the chance that infected individuals will pass the virus to those they live with. Introduced too late, quarantine does little to stop the outbreak.
Results from the study indicated that culling of birds is effective but only if done before the virus infects a human. If a spillover does occur, timing becomes critical. Isolating infected individuals can halt the virus at the secondary infection stage, but once tertiary infections appear, containment becomes challenging without more stringent measures.
Dr. Seema Lakdawala, a virologist, emphasizes the necessity to remain vigilant, as the potential for H5N1 to reassort with existing strains could lead to unpredictable seasonal epidemics. The simulations, according to the Indian researchers, can be continually updated to provide actionable insights for public health officials in a real-time context, thus aiding prevention strategies for potential outbreaks.
