Internal parasites vary in size from large but often relatively innocuous tapeworms, to microscopic but potentially deadly single celled protozoa.
Many intestinal worms have life cycles involving migration of microscopic larvae through the body. Whenever this happens the immune system develops defences and produces antibodies. The consequence is that these parasites are a problem either with the very young, who have not yet produced antibodies, or with the immune-compromised, those with immune systems that have failed. Other large parasites such as tapeworms, that only live in the gut, never stimulate an antibody response. Consequently cats and dogs can get re-infested each time a tapeworm egg is consumed.