Daily updating of your virus definitions should be done. Either by allowing the Antivirus application update itself or manually updating. It’s been estimated that there are 200,000 new variances of viruses and malware showing up every day. That’s like 200,000 new con-artists trying to get your money each day.
The updates to the antivirus definition file is similar to the finger print database the police have access to. As new crimes are committed, finger prints found at the crime scene are logged into a central database and able to be found later on if it comes up. Same with viruses. If your antivirus program knows what to look for, it can try to block it. If it’s never heard of the virus or seen its “finger print” it doesn’t know it bad for you.
Normally it’s little risk however; when the two parts meet: no virus definition and that particular virus attacks your computer, then you have a problem.