It is my understanding that the word parasite is spelled with an i and not a y. However, the title of the anime and manga, when translated into English, uses a y in the spelling.
However, this only seems to apply to the title, since the species is referred to as "parasites" (at least throughout the wikipedia article) and Uda Mamoru calls his parasite as Parasite in episode 7:
Is there a reason for this misspelling?
Or is there maybe a difference between "parasyte" and "parasite"?
According to Wikipedia user Doceirias who translated the Del Rey version of the manga:
Essentially, the word kiseiju1 is consistently translated as Parasyte, while the word kiseichu2 is translated literally as parasite. The creatures themselves are referred to either as kiseichu or parasaito. The word kiseiju only appears one time (if memory serves) and is used to refer to human beings. I decided the distinction was an important one, and that the word Parasyte actually referred to mankind.
As with writing byte instead of bite, Parasyte is a deliberate respelling of parasite for distinction purposes.
1 kiseijuu 寄生獣: parasite beasts
2 kiseichuu 寄生虫: parasite bugs
I believe it is intentional.
Because there's no "Kiseijuu" word in Japanese.
I mean the Japanese original title, "Kiseijuu," itself is a coined word; parasitized + beast.
Japanese people know well about the word "Kiseichu", that means parasite.
Here, "Juu" means animals especially something dangerous or beasts, and "Chu" is bugs. I think this is why "Parasite" won't suit for English title.
And the below is a spoiler for this topic.
In this series, the word "Kiseijuu" will be used or appeared only once. And that is not refer the parasitic monsters like Migi, Tamiya, Shimada, etc.
One of the key person in this series says "Human-being is the "Kiseijuu (parasyte)" who destroy the Earth!"
Parasyte is more like parasite-scythe. Scythe is the weapon of the parasites in this anime/manga, so it's that kind of a pun. Parasyte is because the weapon of the parasites are scythes.
Actually the likely reason it's titled as Parasyte with a Y instead of an I is likely a play on words having to do with a physiological reference to the blood brain barrier (which protects the brain against toxins/pathogens). There are cells called "pericytes" that regulate blood flow through capillaries. So the name Parasyte is likely a clever combination (and way to avoid copyright issues) of the parasite taking over the host in the anime and pericytes that help with brain function. I should note I'm a med student so this came to me while I was doing an assignment lol I'm sure other people have thought of this too though.
Last i checked it was extra little bit of protection against copy-right claims as part of the localization, that turned out to not be required, but was kept anyway.
