Disclaimer: I am not a veterinarian, and have no experience of being a veterinarian. My advice is purely my advice and should not undermine a veterinarian's. If your guinea pig becomes ill, heed the advice given, but if you think the problem is serious, do not hesitate in getting it to a vet.

The first sign of illness is either if your guinea pig is refusing to eat or drink, or if it is huddled up in the corner of the hutch. As they are herbivores, they are more or less constantly eating and their food metabolises very quickly. If the guinea pig does stop eating, it should be considered a very serious condition, as the guinea pig will stop digesting and starve in a matter of hours.

White, sludgy urine - this is not greatly serious, as it is mainly caused by too much calcium in their diet. This, in the long term could be serious, as it leads to bladder stones.

Sniffling, wheezing, constant sneezing, runny nose - your guinea pig probably has a bacterial infection or other illness. Separate it from any other guinea pigs you might have immediately so they don't catch the disease. If it doesn't clear up on its own in a day or two, take the guinea pig to a vet because it might need to be given antibiotics before it will get better. Make sure your vet never prescribes Amoxicillin, because it's deadly to guinea pigs and some vets don't realise this. Make sure it has plenty of water and that the room is kept at a constant comfortable temperature, neither too warm nor too cold.

Blood in urine - this is a symptom that could indicate any of a number of diseases, some of which are extremely serious. Take him/her to a good vet instantly!

Diarrhoea - if you have recently fed your guinea pig a new type of vegetable, or an unusually large quantity of fresh vegetables, that may be the cause. Try not feeding that new vegetable (or not feeding so many vegetables) for a day or so to see if the problem clears up. Whether or not his/her vegetable consumption has changed, if a day passes and your guinea pig still has diarrhoea, take him or her to a vet. It doesn't take long for a small animal to dehydrate and die, so diarrhoea is a very serious problem.

Scratching - as with sneezing, some scratching is completely normal. Guinea pigs spend most of their time grooming themselves. However, if the places being scratched are becoming raw or sore, or losing their hair, the scratching is probably excessive. Your guinea pig may have some kind of parasite, such as mites, or fungus, such as ringworm. Take him (or her) to a good vet, who should be able to run tests and find out what is bothering him. Ifyour guinea pig is kept on softwood bedding, like pine or cedar, he may also be scratching because he is allergic to the bedding. Try changing to non-allergenic bedding.

Trouble walking (stiff joints or stumbling) - this could indicate a vitamin C deficiency. Give plenty of the high vitamin C vegetables listed in the feeding section (even if you have to go out to the supermarket and buy them) and see a vet right away. Your guinea pig may need to have a vitamin C injection.