Parasitic worm colonies are known to invade and castrate a tiny California horn snail, spawning thousands of tiny soldiers that take up 25% of the snail's body weight. These tiny warriors could actually revolutionize how we fight infections in humans.
As much as these colonies are bad news for the snail - at least if it was ever planning on settling down and having some slow-moving kids - the worms also provide some major benefits. The colony's warrior caste becomes de facto white blood cells for the snail, seeking out and killing any unrelated parasites, known as flukes. When the warriors find the new invaders, they use their incredibly powerful mouths to bite and kill the enemy.
Researchers are optimistic that these worms can be used to fight infections in humans. After all, the parasites don't seem to actually harm their host other than the whole castration thing, which almost certainly wouldn't happen in humans many orders of magnitude bigger than their usual host snails. Considering there are 200 million cases of blood fluke diseases every year, finding a simple biological method to control these infections would be a huge boon to the medical community.
The worms reproduce asexually, making them essentially clones of one another. There actually isn't any genetic difference between the soldier and reproductive worms, although they end up looking massively different. This makes them even more closely related than the genetic "sisters" we see with worker and queen bees, and it makes the creation of distinct social castes among the worms even more remarkable.