incrediBILL - 5:05 pm on May 20, 2011 (gmt 0)
It is not a virus. It is scareware and has to be installed manually.
Many viruses have to be installed manually as well, such as clicking the file attachment in the email.
It's more often human stupidity spreading a virus or a botnet than an actual OS vulnerability because people are stupid, naive and greedy and will often fall for anything like "click here to get your $500 Walmart gift certificate"