Back in 1992, when I was more concerned about my acne breakouts and being selected for the Junior cricket team, a freshman at Purdue University was studying the impact of the 1988 Morris Worm event and how it brought about unwarranted changes on Unix systems as it propagated across the network, resulting in the first Denial of Service (DoS) attack. He quickly realised that it was hard to know when a system was infected and what had changed, and by the time the change was picked up, it was often too late.

Source: Tripwire – The State of Security