Scurvy
Scurvy is caused when guinea pigs don't get enough vitamin C. some symptoms are reluctance to move pain in their joints difficulty eating and lumps along the ribcage.
Teeth Problems
If guinea pigs don't chew on blocks they can get sharp points on their teeth that have to be filled off by a vet.
(this is an incomplete list talk to your vet if you have any more questions about heath problems)