Illness mainly. Many pigs get sick and owners may not notice they are that ill until it's almost too late to save them as pigs are very adept at hiding illness. Some illnesses that pigs can get are URI's (Upper Respiratory Infections, they can be fatal in themselves) and sometimes it can progresss to Pneumonia, UTI's (Urinary Tract Infections), bladder stones (also stones get caught in the ureter and urethra). If stones aren't removed it can kill a pig. Mites can kill if they get really bad and the pig doesn't receive treatment. There are pigs we call "heart" pigs. They have heart problems and it's often hard to diagnose a heart pig.
Signs to look for in an ill pig are crusty eyes, crusty nose, runny nose, runny eyes, difficulty breathing, labored breathing, clicking noise in the chest, wheezing, lethargy, not eating or eating very little, abnormal poos (no poo, small, very dry poos, wet runny poo), blood while peeing, squeaking or squealing while peeing, head tilt (indication of an ear infection).
If you notice a pig who is lethargic and off their food/hay even if there are NO other symptoms, start handfeeding immediately with a pellet mash or critical care and start giving water/plain pedialyte and get them to a vet immediately as a pigs digestive system can shut down within 12 hours and they can die. If you notice any of the other above signs, get them to a cavy savvy vet immediately.
To learn more about illnesses visit
www.guinealynx.info.