Whenever possible, it is wise to limit the number of new pigs that make their way into your stock. Even if you commit to trying to have manageable numbers of only animals you breed to fulfill your needs, sometimes adding a pig or two is hard to resist. Avoiding new acquisitions is not a long term reality, however, so there will probably come a time when a new pig enters the fold and safety precautions during introduction will have to be taken.
The first step in bringing another pig home is purchasing your new addition from a reputable source. Breeders who have herds with high health statuses are the best place to start as their standards of care will be passed on to you through your pig. When buying more than one pig, it is also good to stick to the same source for those purchases as pigs in the same herd (herds free of brucellosis are important) have been exposed to the same things. The primary reason a single source is important is because each pig carries bacteria and viruses to which that pig is immune. Pigs from the same source will have exposure to the same bacteria viruses, which means only that set will be introduced to your current pigs. If you buy from two sources, that now means two sources of bacteria/viruses will be joining your heard rather than just one. Sale yard pigs who have traveled and mingled with other pigs can be a great source of swine dysentery, scouring, and pneumonia and can be a roll of the dice even if they appear externally healthy. Purchasing from a reputable source and taking your pigs directly home is the best course of disease and illness avoidance.
Once you've selected your new pig(s), the next step is to quarantine. They should be kept at least 100 yards (or preferably more) from the rest of your herd for 30-60 days and you should cleanse yourself after contact with them and before contact with other pigs. Separate equipment should also be used in the handling of new pigs. During the first two weeks you do not want to treat pigs with antibiotics as that might suppress potential disease and it is far better to let anything that might reside in your pig to show itself so it can be properly addressed. Healthy pigs may still be harboring an illness that has simply not had adequate time to incubate and show outwardly. After these two weeks pass, you can slowly introduce home-produced finisher pigs that will be slaughtered to see if they develop signs of illness. This will also introduce your new pigs to bacteria and germs on your property so they can begin building immunity. If all is well at the end of the 30-60 day period (the exact duration should be determined by your veterinarian), a mingling of herds can gradually begin.
Even when every possible precaution is taken, things can still go awry. To increase your odds of success, be sure to have solid fencing in place that will prevent pigs from escaping and making contact. Also be sure to minimize visitors and ensure that anyone you do allow near your pigs is free of contamination from an outside source. Don't forget to disinfect any equipment that must be shared and keep good control over any rodents or vermin that may attempt to frequent your farm. With a perfect quarantine, things can still go wrong and disease may spread. Even so, quarantine is your best chance at mitigating the risk of new diseases and is a precaution that should be taken with every new pig you bring home.