Indian scientists have simulated how bird flu, known as H5N1, could spread from birds to humans and cause a health crisis. The deadly virus infects birds widely in South Asia and has occasionally infected humans since the late 1990s. World Health Organization data shows 990 human H5N1 cases across 25 countries from 2003 to 2025, with 475 deaths—nearly a 48% fatality rate. In India, the virus recently killed three tigers and a leopard in Nagpur. Human symptoms include high fever, cough, sore throat, muscle aches, and sometimes eye infection. The risk to humans remains low but authorities watch for changes that could make the virus spread easily. New research by Indian scientists Philip Cherian and Gautam Menon from Ashoka University uses a computer model called BharatSim to simulate bird flu spread in a Tamil Nadu village. This village in Namakkal district, a major poultry area with 9,667 residents and 1,600 poultry farms, helped mimic real life. The model shows bird flu starts with an infected bird passing the virus to a human, often a farmer or market worker. The risk becomes critical if the virus begins human-to-human spread. If cases rise beyond 2 to 10, the virus likely spreads beyond close contacts, making containment very hard. The study found quarantining households when two cases are identified can almost stop the outbreak. But if action waits until 10 cases appear, the virus spreads widely and uncontrollably. Bird culling helps only if done before human infection. After human cases start, isolating infected people and quarantining households can slow spread, but tougher measures like lockdowns are needed once the virus reaches the tertiary stage (contacts of contacts). Vaccination raises the threshold for virus spread but does not stop immediate household risks. The researchers warn quarantine too early may increase infections inside homes, while quarantine too late fails to control the outbreak. Experts like virologist Seema Lakdawala note the model assumes very efficient flu transmission and real-life spread may vary. She says H5N1 could cause disruptions like the 2009 swine flu pandemic, but known antivirals and vaccines could help if used fast. Indian researchers say their model updates as data improves, offering valuable guidance to contain bird flu early and avoid a global health crisis. "The threat of an H5N1 pandemic in humans is a genuine one," Prof Menon told BBC, "but we can hope to forestall it through better surveillance and a more nimble public-health response."