The following are signs of a serious problem:
- Not eating
- Crusty eyes or nose
- Wheezing or labored breathing
- Sitting hunched in a corner
- Bleeding from any opening
- Blood in pee, squeeking while peeing
- Severe diarrhea
- Severe injury
- Head tilt while walking
- Drooling, weightloss
- Delivery problems
- Hairloss (can be serious)
For a more complete list of the signs of illness and more information on what may be wrong and their severity, please check the Emergency Medical Guide.
And for help locating a good vet, read Finding a Veterinarian. Each guinea pig owner is advised to find a competent vet before you need one. Be observant and weigh each of your pigs on a regular basis (once a week) in order to catch some of these problems before they become life threatening.
The above information is accessible from the index at Guinea Lynx. A link appears on every page of the Forums (check the buttons on the top right).
Both Guinea Lynx and the Forums can be searched for information -- Guinea Lynx, by using the search feature at the site (on the side bar) and the Forums by using the Search tool (a button appears at the top of every forum page).
For general care questions, you will find lots of information here:
Raising a Healthy Guinea Pig
gl/normal.html will give you an idea of what is normal for a guinea pig.