Neonatal and post-weaning losses represent significant economic loss to the pig industry and in terms of animal welfare the loss of one in ten piglets within the first four weeks of life seems unacceptable. Many losses occur within hours of birth due to a variety of causes, such as overlying, chilling, starvation and disease. More losses occur in the first few days after weaning and the principal cause of these losses is disease. Enteric and systemic infections are therefore responsible for many of the deaths that occur in the early weeks of life. In addition to the pathogenic density in the environment the piglets immune responses determine susceptibility or resistance to disease challenge.

Passive and active immunity are related in a number of ways in their expression but both are profoundly affected by the various stressors that piglets are confronted with in the first few weeks of life. Stress stimulates increased concentrations of plasma Cortisol and this in turn depresses the various components of an immune response.

Amongst the stressors, weaning, mixing into new social groups and relocation may have the biggest effects. In modern systems of pig production piglets are weaned at 3-4 weeks of age and there is constant pressure on the use of expensive buildings, there is often a compaction of these stressors.