Others are not diligent enough about searching for and downloading the latest virus definitions. Smart hackers bank on the negligence of others. Limit port access. Although just about any application that uses TCP requires a port, you can minimize exposure by limiting the number of ports accessible through a firewall. Eliminate unused user IDs and change existing passwords.
Poor maintenance is almost as dangerous as ignorance.
System administrators should routinely audit and delete any idle user IDs. Avoid the use of SNMP across the firewall. SNMP is an extremely useful feature for recording system error messages from servers and routers, but it can also reveal quite a bit of information about your network. Routers should be checked to make sure they do not respond to SNMP commands originating outside the network. Secure remote access.