Nihilego is a fantastic combination of two terrifying things: jellyfish and parasites. Nihilego’s pokédex entry states “it’s unclear whether or not this Pokémon is sentient,” and that’s as good of a description of a jellyfish as any.
Jellyfish don’t have brains, bones, hearts, or blood. They don’t have a respiratory system (lungs or gills) to breathe, but they do absorb oxygen through their skin. Some are microscopic, and others longer than a blue whale. Some of them are immortal and can de-age themselves. They’re made of 98% water, they barely have a nervous system…and yet these creatures have been around on our planet for more than 650 million years, so they must be doing something right. In fact, jellyfish are the oldest multi-organ creatures on the planet. Despite everything they lack, jellyfish do have some organs: a mouth and a digestive system, and reproductive organs, for example.
There are over 2000 different species of jellyfish, so let’s just narrow it down to those who, like Nihilego, are also parasites: the myxozoans. Myxozoans are in the same phylum as jellyfish, Cnidaria, but are curious little organisms who go one step beyond jellyfish, not even having a mouth or guts. And like many parasites, they literally can’t survive unless they are infecting a host. They do still have stinging tentacles, though, so I guess that’s good.
Myxozoans live inside the bones and cartilage of fish, where they can reproduce inside and cripple their host, or even to some extent, control the fish. Some Myxozoans cause neurological damage that causes a fish to uncontrollably swim in circles.
When their hosts die, myxozoans use their little jellyfish stingers to sting, therefore infecting, a new fish.
As shown in Pokémon Sun and Moon, Nihilego has some adverse neurological effects when it infects a host: namely, what it did with Lusamine. If Nihilego is like a Myxozoan, it needs to bond with a human in order to reproduce: and it likely would have killed Lusamine if it could have completed the process.