Guinea Pigs are prone to the same diseases as rabbits, in particular flystrike and dental disease.
As with rabbits, Guinea pigs whilst feel pain, do not show outward signs of pain so may suffer before being noticed.
Guinea pigs, like humans are missing the enzyme that produces vitamin C which is required by the body. Daily supplements of vitamin C are therefore needed by your guinea pig, best in the form of fruit and vegetables. For example Romaine lettuce, dandelion leaves, tomatoes and bell peppers are all good sources of vitamin C.