@josin,zero day viruses/exploits are very costly/effective aka kind of like nukes & you don't use nukes against local street gangs now,do you?zero day attacks are targeted towards highly valuable targets like govt installations(or in case of China,Dalai Lama offices
),fortune 500 companies etc & there is almost zero chance that people like you or me will ever see a zero day virus/exploit.
always keep your OS updated along with java(better uninstall it unless absolutely necessary),flash(try to use only on trusted site with default behaviour as blocked),MS office & adobe acrobat(my advice ditch it altogether & use sumatra pdf).if you don't do this then no AV can help you.
now-a-days you don't need to download viruses.simply open a page(& i mean just open,no clicks etc) & if your system has some vulnerability(like those i mentioned above) your pc will get infected by an almost untraceable rootkit.