If there's any doubt about the nature of the disease, take your guinea pig to a veterinarian right away! See the previous section for techniques for finding a good guinea pig vet. Sometimes a simple medical procedure can clear up a problem that would otherwise be fatal. That said, here are some common symptoms with what illnesses they may indicate.

Note: This is NOT intended as a replacement for a visit to a reputable veterinarian! The maintainer of this FAQ takes no responsibility for any misdiagnoses that might result from reading this section.

Some sneezing is completely normal, just as with humans. However, if your guinea pig is sneezing all the time, or is sneezing a lot in combination with other symptoms, he or she may have a bacterial infection or other illness (see next).
Sniffling, wheezing, constant sneezing, runny nose:
Your guinea pig probably has a bacterial infection or other illness. Separate him (or her) 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 him to a vet because he may need to be given antibiotics before he will get better. Make sure your vet never prescribes Amoxicillin, because it's deadly to guinea pigs and some vets don't realize this. If the vet prescribes any sort of antibiotic, you should give the guinea pig a supplement of lactobacillus acidophilus (you can find this in health food stores) or live culture yoghurt, so that the antibiotic doesn't kill the good bacteria in the stomach that enable digestion. Also, make sure he 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 right away!
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 diarrhea, take him or her to a vet right away! It doesn't take long for a small animal to dehydrate and die, so diarrhea is a very serious problem. If your guinea pig has been on an antibiotic, the problem may be enteritis, which just means that the antibiotic is killing off the digestive bacteria in the stomach. See sniffling section above.
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. If your guinea pig is kept on a softwood bedding, like pine or cedar, he may also be scratching because he is allergic to the bedding. Try changing to a non-allergenic bedding like the ones on Debbie's list (see the bedding section) and see if this helps.
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 get a C shot.
Loss of appetite:
See a veterinarian immediately. Being small animals, guinea pigs usually eat pretty much constantly and metabolize food very fast, so if an illness or other condition is preventing them from eating they could die overnight.
You will probably need to trim your guinea pig's toenails, unless he or she does a lot of running around on bricks or concrete or other rough surfaces that will keep the nails short. Once the nails start getting long there is nothing but you clipping them to remedy the situation; the nails will eventually either curl back into the pad of the foot, crippling the guinea pig, or else break off and sometimes cause bleeding and infections in the process. You can clip the nails at home yourself or, if you feel insecure about it, you can have a vet do it the first time so you can see how it's done---although they may charge you a fair bit for this. You can use either a normal human nail clipper or the clippers with curved blades they sell in pet stores for trimming cat nails.

The easiest way to do this is to have a friend help you, so that one of you can hold the guinea pig while the other trims the nails. If this isn't possible, some people recommend rolling your guinea pig up in a blanket or something, so he (or she) can't see and doesn't struggle, and turning him on his back in your lap so his face is still covered but his feet stick out. I've never tried this myself, so I don't know the precise logistics of it, but apparently it keeps them from putting up a fuss.

The thing you have to be careful of is not to cut the quick, which is the pink part in guinea pigs with white nails. Just like in humans, the pink part shows how far the flesh of the toe extends, and the white part has no nerves. If your guinea pig has dark nails, you may need to use a brighter light source to see the quick, which should be slightly darker than the end of the nail. If you still can't see where the quick is, just cut the nails often and a little bit at a time and you should be fine. If you do accidentally cut the quick a little and it starts bleeding, dab a bit of hydrogen peroxide on the spot to help prevent infections. Try to hold him or her until the bleeding stops so that the site stays clean and the cut is given a chance to heal over somewhat. There are products---"Quick Stop" is one of them---that you can apply to the site to help stop the bleeding; these are helpful (but not necessary) in a situation like this.

Guinea pigs of both sexes can be neutered, and in some cases should be. There is some evidence that neutering a female can reduce incidence of uterine cancer. Also, if you have an older female that may not have had a litter yet, she should definitely be spayed for her safety. There are no known health reasons to neuter a male guinea pig, although I hear that it can reduce their sex drive and cause them to stop mounting female guinea pigs, if that is an issue.

The risk can be very small if you find a good vet to perform the surgery. You can begin by looking for "exotics" (read: not just cats and dogs) vets in the phone book. Or call ordinary vets and ask who they refer their serious guinea pig cases to. Call around, ask for recommendations, and don't be afraid to drive a long distance -- it's only once, and it could save your fuzzy's life to be at an experienced vet's. Ask any prospective vet how many guinea pig spays/neuters they have done in the past year, and what their success rate is. For a good vet, it should be well above 90%.