The biggest threat posed by the new version of Bagle is its ability to turn off existing firewalls and anti-virus protection.
"Any Trojan horse which turns off your anti-virus or firewall can open you up to further attack, even by very old viruses," said Graham Cluley, senior technology consultant at anti-virus firm Sophos.
"This trojan horse is aiming to take advantage of many people's reflex reaction when they receive an executable file via email. Rather than not touching it with a bargepole, they often can't resist double-clicking on it, even though they have no idea if it's safe or not."
Bagle's quick spread is attributed to its immediate release onto a pre-set zombie network.
"This threat was clearly spread through a deliberate spam mass-mailing. This trend of seeding an infection through spam distribution has been growing during the last year or so," said David Emm, senior technology consultant at anti-virus company Kaspersky.
Late last year SC reported a rise in the Bagle worm. Since the middle of 2004 it has remained one of the top ten most prevalent internet viruses.www.sophos.com