A "good" virus -- essentially, an anti-virus virus -- uses the same techniques viruses do to try to stop a virus or close a security hole. One example was MSBlast.D (also known as Welchia or Nachi). MSBlast.D was a version of the MSBlast worm that actually worked to patch the security vulnerability in Windows that was exploited by the original MSBlast worm. After fixing the problem, MSBlast.D sent itself out to patch other systems. Unfortunately, MSBlast.D turned out to cause more trouble than it was worth as it clogged corporate network traffic while multiplying and scanning other computers for the security weakness [source: Lemos].
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