It was a commonly held belief, and still is in some places, that piglets pick up infectious disease from their dams. It is true for a number of potential pathogens such as E. coli, clostridia, fusiform bacteria, some streptococci and staphylococci but it is not true for many of the more serious enzootic pathogens. These are picked up after weaning from older pigs.

By the time a female pig has reached farrowing age, she has developed a powerful immunity to most of the serious pathogens enzootic in the herd and in most cases has thrown them off and is free from them. She passes the full spectrum of her humoral immunity on to her piglets, mostly in her colostrum and to some extent also in her milk. This makes the piglets immune to the same pathogens. Before weaning, the piglets are either not encountering the enzootic pathogens because the sow is not shedding them, or if they are encountering them, by say aerosol or from other parts of the farm on pig persons clothes etc. they are in low numbers. Their maternally derived immunity can prevent infection.

After weaning, they are exposed to an increasing array and concentration of pathogens shed by older weaned and growing pigs. This is at the same time as they are suffering the extreme stress of weaning, when their maternally derived immunity is wearing off and their own active immunity has not had time to fully develop. As they grow older the total weight of the group and stocking density increases and the need for maximum ventilation grows. This is often inadequate, and enzootic infections particularly respiratory infections begin to cause clinical disease.

So the picture is one of enzootic infections moving down against a flow of piglets coming up.

Methods of disease control such as Medicated Early Weaning, Isowean, Segregated Weaning and Multi-site production make use of these facts to break the cycle of infection.