Anti Virus tools work by scanning files and comparing contents against a signature file. The signature file is the file your AV program updates from the vendor automatically weekly or even more frequently from the vendor. The signature file contains the digital "signatures" of known or predicted viruses and other malware. The bad guys continually modify their stuff so the AV programs can't detect them. When a new version of malware gets into the wild it may be a couple of days before the AV program vendors detect it and update their signature files. So dispite having up to date AV program there may be a window when you are susceptible to the latest version of a virus. Where I work we have some of the most sophisticated protection mechanisms available but a virus still occasionally gets through.