Mac has built in FireWall. It prevents viruses VERY well vut doesn't get rid of it.
This doesn't necisarilly sound like a 'virus' it sounds like an 'add on' that was added to your Safari preferences via downloaded program which you may have downloaded unintentionally- like say you are looking at a minecraft texture pack and you click a button that says 'download' yet it does nothing (so you think) but in reality it's not the true download button but an add that does some crud to your computer. You can check in your Safari precernces and check for anything 'iffy'. Otherwise go through your recently downloaded programs (Mac does this often with Minecraft mods) and uninstall them (then if you are really wanting the texture pack mod etc. download it from a more reliable website.) This is my advice for you I don't really know what else to do but I will research this.
Jacky708