Antivirus firms are raising threat ratings as the virus spreads at up to 50,000 copies an hour.
"We have several reports about a new Bagle-related downloader, which was spammed in emails to a large number of people," said Alexey Podrezov, virus analyst at Finnish company F-Secure on the company's weblog. "So far three variants are reported and they all have the same functionality (they are just recompiled and repacked versions of the original Trojan)."
According to analysis from antivirus companies the virus originated in a Yahoo group and is has been successfully partly because it downloads information from a list of websites from around the world. These sites are governed by different legislation and so difficult to shut down simultaneously.
In March SC reported virus authors were increasingly working together to maximise the effectiveness and make more profit out of their viral creations.
Earlier this year SC profiled F-Secure's Mikko Hypponen, who's job it is to track the deluge of internet viruses plaguing worldwide networks.