Guinea pigs often pretend they are not sick. To the unsuspecting eye, they may look fine, but with more careful inspections you may begin to notice signs of illness such as listening to them breathe and hearing clicks, sneezing, weepy nose or eyes, weight loss, etc. Some guinea pigs are born with heart problems. If the heart problem is never detected, this is something that can cause a sudden death. Or, a major contributor is foods, chemicals, beddings, etc. There are many foods or parts of foods that guinea pigs can't have. Guinea pigs shouldn't be around household cleaners and chemicals and certain beddings can be bad, especially if your guinea pig decides to eat it. Anyways, guinea pigs aren't something you can just bring home, like hamsters. They require LOTS of knowledge on the owners part and care. Many people learn this the hard way with their first guinea pig.