...monitor the system proactively and help manage aspects such as firewall and URL filtering rules and updates to the anti-virussystem. A third party will have invested in skills and capabilities that a small company would find hard to afford itself...
http://www.computerweekly.com/Articles/2004/11/01/206387/secure-state-of-mind.htm
...layer is added when defences are integrated. For example, if a virus is known to open up a back door, the anti-virussystem should not only search for the virus but also for the back door. Alternatively, it must prompt the firewall to stop...
http://www.computerweekly.com/Articles/2004/04/20/201840/Top-five-threats.htm