The people who release the viruses are often anonymous mischief-makers, or 'script kiddies'. That's a derisive term for aspiring young hackers, usually teenagers or students, who don't yet have the skill to program computers but like to pretend they do. They download the viruses, claim to have written them themselves and then set them free in an attempt to assume the role of a fearsome digital menace. Script kiddies often have only a dim idea of how the code works and little concern for how a digital plague can rage out of control. Our modern virus epidemic is thus born of a symbiotic relationship between the people smart enough to write a virus and the people dumb enough - or malicious enough - to spread it.
In related news: German magazine c't apparently has evidence that virus writers are selling 0wned machines to spammers. In this case, "virus writers" probably means "hoodlums who hang around virus writers' forums".
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