Delete the backslash.
I don't know why Cryptosporodium is found with and without backslash (I made a test with a Wikipedia text), but Giardia is found without backslash only.
That's really strange, thank you very much!
Nothing strange here, you are just inserting backslashes at the wrong places.
"\G" is (apparently ) a valid GREP command but "\E" and "\C" are not. The default behavior for surplus backslashes is to ignore them, so these match "E" and "C" only.
However, you should add a backslash before the full stops in your terms, because without, they will match any character. Chances are small you have something like "Ex coli" in your text - but if you try, you will see it gets matched. (And you may add some other GREP rules that do catch something unexpected because of this.)
 It should be something like "start of attempted match", much like the function "\K" that got added fairly recently.
Thank you for the additional information! It's really helpful since I'm not familiar with GREP.
I'm thinking about investing in some lynda.com training or books because it seems like I could have been saving myself SO MUCH time had I been using these years ago.