The Worm Core is one of the chief differences between the Kikaichū and the Creepers. Its a psuedo-organ, which takes shape during the first night following a child being made into a host of the Creepers. However, up until Kaoru Aburame was made into a container, they were completely unaware of the specific requirements of this Core. Namely that in order to become complete and work properly, it needed a substantial amount of flesh, not just any flesh either - it needed human flesh for the purpose of becoming stable. But, in all cases except for Kaoru's, there was no human flesh to be found outside of the container, and thus it would turn upon its very host, leading to their death within mere moments.