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An epidemic often begins as a whisper—in barnyards, wetlands or milking parlours—long before its roar reaches emergency briefings and policy halls. As seen with H5N1 in early 2025, dismissing these quiet signals can cost lives. The virus, once confined to birds, has now infected over 40 mammalian species, each host offering new evolutionary opportunities.1 A single bird-to-cow spillover of a B3.13 clade virus in the late 2023, followed by a separate D1.1 introduction in Nevada weeks later, marked its troubling spread into US dairy herds.2