A range of parasites and mites can affect guinea pigs. The most common are ringworm, fleas, lice and fur mites. Signs to look out for are scratching, fitting and hair loss.
Treatment such as Ivermectin is effective against mites and your vet will be able to recommend an antifungal medication for ringworm. Please note that ringworm is highly contagious to people so appropriate hygiene precautions needs to be taken when handling an infected guinea pig.