It's obviously from a single poster who is in a ton of the main forum threads.
If you click into secondary sections, you will not get that alert. So it is one person with a bad sig link, avatar, or posting up some shit that is infected in the main section. If you would get the same alert in all sections, than you could say it's a GFY issue. But since it's main forum only, it is a single user infecting people and posting a lot.
