On the southern coast of Australia, a long-nosed fur seal has become the first mammal in the country confirmed to carry H5 bird flu, a quiet but consequential crossing of a threshold that biosecurity officials had long hoped to avoid. Found dead near Beachport in South Australia—a state already bearing the weight of 183 of Australia's 298 recorded avian cases—the seal's infection speaks to the pressure that accumulates when a virus saturates an ecosystem. Nature does not observe the boundaries we draw between species, and this moment asks whether what has been contained can remain so.