Although the virus definitions database gets update regularly and automatically, the ClamAV applications do not.

Sometimes it is not enough to just update the definitions. New functionality must be included into the applications to make sure each and every virus flavour gets caught.

Usually, Apple will update the applications for you through a system or security update.

Alternatively, you can download the sources and compile them yourself. Unless you are very familiar with the innards of OS X Server, you should not attempt this before version 10.4.7. A tutorial on this is available here.