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Porcine reproductive and respiratory syndrome (PRRS) and Post-weaning multi-systemic wasting syndrome (PMWS) are two of the most important diseases affecting pig production. They are both viral disease and were first noticed roughly 20 years ago. The initial cases of both diseases followed changes in husbandry and intensification of pig production in both North America and Europe, and it is now thought that specific farming practices led to the spread and subsequent increase in severity of these diseases. The practices included implementation of indoor production systems, increases in herd size and stocking density, early weaning and abrupt changes from milk to creep feed at weaning. These, combined with changes in environmental factors such as temperature and relative humidity in pens that can affect herd hygiene, were key aspects in the emergence of the diseases. It is thought that the PRRS virus and PCV2 (the causal agent of PMWS) had been present in pig herds for decades if not centuries before the diseases emerged, and the viruses slowly mutated into disease-causing forms. In earlier times, pigs were kept on small-scale farm and successful transmission of the disease was less likely, if not impossible.