There are many factors that suppress both innate and acquired immunity levels but in pigs infectious agents are the most common ones. These include:

• Mycoplasma hyopneumoniae the cause of enzootic pneumonia.
• Pasteurella multocida.
• Aujeszky's disease virus.
• Swine influenza virus.
• African swine fever virus.
• PRRS virus.

These agents may destroy the macrophages or lymphocytes, or delay or reduce the efficiency of the immune response, or damage the innate defences.

Weaning time is also a period of immuno-suppression by withdrawal of the sows milk that contains the protective mechanisms of IgA. When pigs are mixed, moved or stressed the plasma cortisol levels rise with a similar effect and the demand for vitamin E rises.