Why is Parasyte spelled with a y in the title?
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