That is, multiple and varied sources show that a word is in regular use and has staying power. If you have never heard the word parasital used in context, that's fine. However, the grammarians at Merriam-Webster have heard it in context. Many times. Enough to judge that it is an established, current, and meaningful inclusion in the language.
Noted and rejected, the proper usage of the word as defined specifically by the RACGP is the final word on the matter. You are free, of course, to argue the proper terminology as defined by the foremost medical authority.

Discussion OVER.