The global expansion of chicken farming has created conditions that allow Campylobacter to spread, mix and adapt more readily. Expanding flocks transformed bacterial spreadThe worldwide chicken population has grown seven-fold since the 1960s, reaching approximately 31 billion birds. “As chicken populations have grown, bacteria that were once largely confined to wild birds have gained far more opportunities to enter poultry flocks, spread and become established. Dense flocks can amplify new strainsThe findings suggest that enormous, densely packed flocks may function as ecological ‘pathogen sponges,’ taking in bacterial strains from different sources and amplifying them. Mathematical modeling showed that after chicken populations pass a critical size, Campylobacter strains originating in wild birds can continue circulating within poultry without repeated introduction.