It is probably a trojan rather than a virus. I rarely see viruses these days. You can get a trojan from anywhere not just 'dodgy' sites. In unknowingly accepting the trojan it bypasses your firewall and your AV as you have, in effect, given it permission to run and install. This is why I never used a firewall for 5 years; if you are on BB you do not need a firewall (especially if you have a router) just some basic common sense.
Every computer that I have to repair which have Trojans, have up to date firewalls and AVs, so the shed loads of 'lab tests' are somewhat ,meaningless.