Bagle was first detected in 2004 and was another well-known type of malware used for mass-mailing.
A rather complex virus, Bagle infected users via an email attachment.
Unlike other mass-mailing viruses of the time, Bagle did not rely upon the MS Outlook contact list in order to generate more locations to send itself to.
Instead, it harvested email addresses from various document files that it found on the infected computer, including MS excel files and even email addresses stored in text editors.
Perhaps the most dangerous part of Bagle was the fact that it was able to open a backdoor on a computer, allowing a remote user to gain access and control of the infected computer.
From this point it could then download additional software which could be used to either spy on the infected machine or to steal important information.
Additionally, it could also be used to launch DDoS attacks to targeted networks, computers or servers.
The original version of the Bagle virus was designed to stop working after January 04 but there were many other variants created which are still out there today and still causing problems for some users.
Read more : Malware That Changed The World