Usually, the first signs of illness in your guinea pig will be a loss of appetite and lethargy. Although not all illnesses affect the guinea pig in the same way, if you see your guinea pig huddled in the corner of the hutch, refusing to eat or drink, it is almost always indicative of a serious affliction. Being herbivores, guinea pigs eat almost constantly, and metabolize food very quickly. If, for any reason, they stop eating, it should be considered a very serious condition, as the guinea pig could die in a matter of hours. Some illnesses can also cause diarrhoea, which is even more serious. Diarrhoea can dehydrate a guinea pig very quickly, which can also cause death if not treated right away. Some illnesses, however, start gradually with a few symptoms and then progress to the more advanced stages. These symptoms can include wheezing, discharges from the eyes or nose, and hair loss. Other afflictions, such as cysts or bladder stones, are not necessarily related to a particular illness or infection, and have their own symptoms that may be more difficult to spot on casual inspection. As a general rule of thumb, watch your guinea pigs regularly, and get to know their normal behavior patterns. If you start to see behavior that differs from the usual routine, then it could be a sign that an illness is setting in. Watch their droppings and their urine for deviations from the norm, and keep an eye on their daily appetite. Theres no sure-fire way of detecting illness, and not every symptom is the sign of a disease, but its always better to err on the side of caution.