No matter how good your antivirus software is, (and yes this includes all the major brands) viruses can get past them. Usually this is done by just simply being new. Like many other anitviruses, Norton needs to download new virus definitions in order to find new viruses. There is a delay between when a virus is released and eventually discovered (that depends on how malignant it is) until it is finally processed so you antivirus knows what to look for. Once again, antivirus software is no substitute for the application of caution and common sense.