
alot of viruses these days are FUD ( Fully Un-Detecable ) Runtime + Scantime which means your anti-virus will not detect it at all (or atleast till its been run through virustotal a few times ), you would actually be suprised how many people get infected and never realise at all or not until 3 months down the line when all there sh*t gets robbed or they get passed around to other people as installs.