antivirus for webserver

[Laura Arjona Reina <laura.arjona@upm.es>]

Hello
I have a debian etch webserver, it only has installed ssh+apache+php+mysql.
It has no GUI.
Nobody sits or connects there to work, only administrators for backing 
up and update the system.
I tried to secure it installing and configuring bastille.
Now the only open ports are 22 for ssh and 80 for apache.
We don't need any other service: no DNS, no email server, no ldap, just 
webserver.
The connections to mysql are closed from outside, only the webserver can 
access mysql databases.

My question is if it is needed to install an antivirus for keeping the 
webserver safe. And if it is needed, which antivirus could I use?
I thought about clamav but I read about problems keeping up-to-date the 
software shipped with etch-stable.

Thank you

Laura Arjona