The virus, a new version of Fbound, appears as an e-mail with the subject line: "Patch" and contains a file called patch.exe.
David Emm, product marketing manager at antivirus vendor McAfee, told CW360.com the virus originated in Japan. He said: "It doesn't appear to do any damage but it could flood your e-mail gateway and so disrupt business."
Antivirus firm MessageLabs reported that at 10am this morning there were 1,631 infections, mainly in the far east, with the virus spreading towards Europe.
Another antivirus firm, McAfee, said Fbound runs as a mass-mailing virus, like the notorious Love Bug, which sends itself to all users found in the Windows Address book using SMTP (ie e-mail). McAfee has issued an update to its antivirus software to detect Fbound.
However, MessageLabs said the format of the e-mail could cause additional problems for mail gateways running certain antivirus software.
The company said the e-mail attachment in Fbound is encoded in a single base 64 encoded line, several thousand characters long, which could make detection tricky. Additionally, it noted that the worm might also be truncated or corrupted by other mail gateways, which cannot cope with lines of this length.