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Early-weaning at an age of less than 21 days and removal of pigs to a second isolated site, which is commonly referred to as segregated early weaning (SEW), has been shown to substantially reduce disease transfer from the dam. This strategy has been successful in reducing the number of pathogens, but has not been successful in eliminating all pathogens. Although SEW has failed in most instances to totally eliminate pathogens, performance as measured by gain and efficiency through the nursery phase has been shown to be enhanced. In addition, SEW pigs have been shown to perform well on less complex nursery diets. Pigs which are continued on a similar isolation regime to market weight have been shown to have a remarkable improvement in feed intake, gain and efficiency. However, pigs which are co-mingled with conventional pigs after the nursery phase have either no improvement in performance or reduced performance. Backfat and carcass lean yield have been shown to be enhanced by SEW in high lean gain pigs, but not in lower lean gain pigs. Exposure of pigs to antigens which activate the immune system and increase the level of immunological stress has been suggested as the mechanism involved in depressing growth and performance of pigs.