External (fleas, lice, mites) and internal parasites (hookworms, lungworms, roundworms and tapeworms) are found in and on companion animals world wide. Infestation with external parasites can cause severe itching and hair loss while signs of infection with internal parasites depends on the organ system affected and, in many cases infection, may not lead to any clinical signs. Some of the most common parasites of dogs and cats are roundworms which can also infect humans, especially children, which can cause quite severe infections.