virus bypass av and firewall

I got a virus the other day and was forced to format my computer. So I got a few questions..

First, how can I get infected with a virus just by visiting a website?
Second, how can that virus bypass my firewall and antivirus?

I am just curious.. I tried googling it but no results. Hope you experts out there can enlighten me..

 

One thing is older programs installed. Java comes to mind. Get an update to Java and don't un-install the old one. It's still in there so someone finds out how to hack the old one. Lets things in without you knowing of it @ 1st.

 

Malware is getting sneakier everyday. Just loading the site transfers information to and from your computer. That's the nature of the beast.

If you browse somewhere you are creating a hole in your firewall for something to get through by the very act of surfing.
No antivirus is 100% effective.

How to Protect yourself from Malware!

 

Cross site scripting (XSS) is the biggest concern when visiting any website, the site itself may be safe but any of the pages could load adverts, pictures, etc. from off-site. If any of the linked items are scripted to load a piece of malware that your software is vulnerable to, or you're running from an administrative account, you're in trouble.

Keep your software updated and block, or at least control, any scripts loading from sites away from the one you're visiting. NoScript is available for Mozilla-based browsers, I believe there are others available for Chrome and Opera too.