Virus scanners with greater amounts of signatures can detect greater amounts of viruses. You have to update your virus scanner because new viruses come out all of the time. Your virus scanner that you installed in 1997, chances are, will have no clue about viruses released last year, much less last month. If you have an old or non-updated virus scanner, you may think that you are protected, but it is just like closing the doors to your house but keeping the doors unlocked. Check your scanner's website or contact their technical support.
Many companies have virus-updating policies.
They make sure that their signatures are updated on a frequent basis. This is good, but, unfortunately, most businesses like these update their signatures once a month. This used to be acceptable, but I believe this practice cannot continue any more. You should update your virus scanners at least once a week. Yes, let me repeat myself.