The way I understand it is it is like peeling an onion. Some wont need more than one round if just doing maintenance. Every time you use humaworm, the product is able to peel that layer of parasitic infection and work deeper the next time. In a literal sense, the first round might just get the small larvae and smaller parasites, but once those are gone, it goes to town on the next stronger ones left. Anyone else please feel free to correct me if I am misunderstanding it.