Limiting disease introduction and spread within a herd can be achieved by using four principles. The first principle is to understand the disease by knowing as much as possible about things like how it spreads, how long it stays in the environment, under which conditions it thrives etc. Also part of this principle is keeping track of what it is doing in the herd, and this can include monitoring and diagnostic testing. The second principle is to keep away new diseases, or new sources of the same disease. This relies on good biosecurity, and even the best barn will have areas they can improve. Thirdly, replacement management can reduce disease transmission. Ensuring gilts have a strong immunity before breeding will help reduce the possibility of spreading disease to their litter. Having gilts enter the herd earlier, closing the herd, purchasing gilts that have been exposed and developed immunity to certain pathogens, like Mycoplasma, and purchasing gilts from PRRS-negative herds can all help depending on the specific barn conditions they are entering. Finally, good husbandry and management protocol dedicated to reducing transmission can help reduce the impact of a disease.