A new computer virus is spreading across the internet via e-mail and file-sharing programs, computer security experts have warned.
The mass-mailing worm, dubbed Fizzer, is rapidly infecting computers using the Windows operating system in Asia, Europe and the US.
Experts say it is a complex virus which can disable anti-virus software, steal passwords typed on a keyboard and even open a back door to a computer.
People are advised to update their anti-virus software and be wary of e-mails from unknown sources.
We've upgraded it to high-risk just for the fact that we've seen so many in the last day
Mark Toshack, MessageLabs
The Fizzer worm has been described by experts as a particularly unpleasant strain of a mass-mailing virus.
It was first detected by e-mail security company MessageLabs on 7 May but was considered to be of low risk to computer users.
Since then, the bug has used a variety of tricks to multiply across the internet. MessageLabs has recorded 17,765 cases in 24 hours in the UK alone.
"We've upgraded it to high-risk just for the fact that we've seen so many in the last day," said MessageLabs virus analyst Mark Toshack.
Other anti-virus firms have also issued similar warnings, with Finland's F-Secure classifying Fizzer on the same threat level as the Nimda bug.
Fizzer spreads through file-sharing programs such as Kazaa as well as by e-mail containing a file attachment with a .exe, .pif, .com or .scr extension.
Once a computer is infected, the virus will scan the victim's address book and send out infected messages using different subjects, message texts and file attachment names.
It also installs a keylogging program to record every keystroke, as well as opening a way to access a victim's computer over Internet Relay Chat.
Additionally, the virus regularly connects with a web page to try to download an updated version.
MessageLabs said that although Fizzer was not, at the moment, as prevalent as other viruses like the Klez virus, it was likely to be around for a long while.