Australia, long insulated by geography from the H5 bird flu strain that has swept the world, has crossed a threshold it cannot uncross. On Kangaroo Island off South Australia's coast, four suspected cases this week suggest the virus is no longer arriving from distant shores but circulating among the continent's own wildlife. The island shelters endangered sea lions found nowhere else on Earth, and nearly all of Australia's mammals and birds are endemic — creatures whose evolutionary isolation may now render them uniquely vulnerable to a pathogen they have never encountered.