You have a computer and you have a Security Suite installed. The next thing you know you are infected, your Security Suite is unable to run, and your computer is rendered useless. So what happened?

Your house has an alarm system. One day a gentleman dressed as Police Officer knocks on your front door. Talking through the door he tells you that as part of a crime prevention initiative they are  providing free security inspections. This gentleman looks like a Police Officer and sounds like a Police Officer, so you disable your alarm system and let him in. The Officer asks to look at your safe and you open it for him. He tells you that all of your firearms and valuables need to be secured, and he will show you how to secure them. So now you have a gentleman who looks like a Police Officer in your house, the door is locked, the alarm system is disabled, all of your firearms are locked in your safe and he is sitting at the kitchen table chowing down on the meatloaf you just cooked and sipping a frosty beverage. You notice that the label on his shirt says "Joe's Costume Rental".

The individuals who write viruses are smart enough to know how a computer communicates and functions. They know how anti-virus programs work and the surfing habits of the general population. As soon as a virus is released it will go to work. In a matter of hours, (maybe days), it will have infected a number of systems. A few more hours/days, will see a solution to the removal of the virus, and a few more hours/days yet, the virus will be labeled and most Anti-virus software will be aware of it and able to block it.

This, in small part, is why your system can be infected by a virus despite an updated anti-virus being installed on your computer. None of the vendors on the market today will provide a protection guarantee, mostly because it is nearly impossible to guarantee protection.