Somewhat off topic but some may find this interesting. A virus, any virus relies on propagation. The life cycle of a virus is sort of two steps. Initially a good well written virus will have no problem doing damage and propagating, once it is discovered and patches, fixes released and AV software updated it will begin to die. Both the I Love You and Melissa viruses are good examples of viruses that within hours of release infected millions of systems. Hey, email is an excellent means of propagation. People will open anything.
However, discounting email and basic viruses there is a much deeper threat to what should be secure systems. People are their own worst enemies. I see and read security bulletins almost daily and this one I read sticks in my mind. As a test a series of DVD disk and USB Thumb drives were infected with some script. Nothing malicious or destructive but designed as a test, let's say they would "phone home". The DVDs had catchy titles and were printed nicely. They looked real official. They were intentionally left out in the open around assorted DoD (Department of Defense) and DoE (Department of Energy) facilities, this pertains to the US. The number of people who picked these things up and shoved them in classified system computers was astounding. Something like 80% were actually run. Go figure huh? My work system (networked systems) actually have the auto run disabled. I can manually run an executable. For these disk or thumb drives to run, people had to manually run them. Go figure.
Ron