I use Clamav for my antivirus in all of my Slackware box (laptop and 2 desktops). On every machine, i set the update mechanism differently. For my desktops, i set it automatically update every 15 minutes and one hour because they are all connected to the Internet, so it's not a problem to use this settings, but for my laptop who doesn't connected all the time, this could be problem (I can also set it to become automatic, but it would be a waste), so i make the update process manually.
The problem arose when i tried to update my database from my campus network. When the traffic was so full, the update process failed all the time and causing Clamav to blacklist the mirror. Unfortunately, the mirror Clamav blocked was the primary server, so when i tried to update the virus database at home, it always failed. I even tried to re-install the apps, but the problem persists. The problem is already listed on the Clamav Support page, so by just doing what it's written on that page, i'm able to download the update again.
It seems that Clamav will blacklist a mirror when you have a bad network and update process failed all the time. If you are sure that your network is good, then find mirrors.dat and remove them. It's Clamav's file that store the mirror information and the status (normal or blacklisted). After you deleted them, Clamav will create a new one and use the default server to download the updates.