Actually this is a 'worm', not a virus. Once a worm penetrates another computer it continues seeking access to other areas. Worms are often equipped with dictionary-based password crackers and other cracker tools which enable them to penetrate more systems. Worms often steal or vandalize computer data. Viruses spread through programs that are shared with other computers over the Internet.
Some viruses are malicious and damage files and programs.
Viruses spread without the user's knowledge, and should be scanned for by an AntiVirus program on a regular basis. This worm and other trojan-horse type programs demonstrate the need to practice safe computing.