This variant of Sober generates e-mails that purport to be from the CIA or FBI. These messages tell the recipient they have been looking at illegal Web sites and should answer some questions in the e-mail's attachment. If the attachment is opened, the computer is infected, and the virus sends copies of itself to any e-mail addresses found on the hard drive.

Allan Bell, the marketing director at McAfee Australia, said that over the past 24 hours more than 90 percent of all virus laden e-mails monitored by its partner Postini contained a copy of Sober. "(Sober) was generating around 15 million out of 16.8 million (virus-infected e-mails), so about 90 percent of the traffic is this particular virus," Bell said.

Bell called the virus "prolific," saying it is capable of generating large volumes of traffic. That flood could slow or even overload many e-mail gateways, in a way that resembles a denial-of-service attack, which attempts to overwhelm a targeted system with excess data requests.