It has been a year since the bird flu tore through the Midwest: enough time for decimated farms to cash their indemnity checks and begin buying replacement birds; for the wholesale price of eggs, which doubled, to slide back to normal; for national awareness of the outbreak, the worst animal-disease epidemic in United States history, to dissipate. But among the poultry farmers who endured the flu, and others watching elsewhere in the country, there is a pervasive uneasiness, because after a year of scrutiny, federal and academic scientists still cannot say for sure how their properties became infected. Despite their own efforts to harden their defenses, and new federal plans to help them, it is possible that poultry farmers are not equipped for the flu to return among the United States’ billions of chickens; and that ranchers and pork producers might be equally unprepared if an unfamiliar disease detonated among the country’s 92 million beef and dairy cattle or 68 million pigs. Planning for epidemics, animal or human, is to a large extent based on what a disease did the last time. It is much more difficult to predict what a disease will do next. ... Naturally occurring epidemics might be as dangerous as maliciously introduced ones; and diseases in plants and animals could undermine national security as seriously as human epidemics would. ... A study done in July estimated that the flu cost the United States $2.6 billion in lost sales, almost $400 million in forgone taxes and 15,693 jobs. But as devastating as the losses were to Iowa, Minnesota and other states, their 50 million turkeys and hens represented a small portion of the poultry industry. The largest concentrations of chickens in the United States — Georgia, which raises 1.3 billion broilers a year, and the Delmarva Peninsula, where the broiler business began in the 1920s — lie under another migration route that ducks take over the Americas. If the flu were to land in those areas and spread the way it did in the Midwest, much of the United States poultry industry would be in jeopardy: almost nine billion birds, 90 billion eggs, more than $40 billion in earnings, 1.3 million jobs.