What’s going on with the word “virus” and the tags

This is weird. I have a podcast about technology and the last week we talking about virus and antivirus. Well, when i add the tag “antivirus” into the box at the right from the editor page the word added fine but when i try the same with the word “virus” nothing happend.

When i write the word “virus” WP show me the suggestion “antivirus” that i add first but dont let me add my own word still in the tag field (even pressing the button). I added manually in the tags section but when i try to select again from the suggestion menu dont select it.

What’s going on with that word ???