The US State Department has admitted that it has been infected with the Klez virus.
Hundreds of rogue emails have been sent to victims, appearing to be official material from one of the Department's offices.
A spokesman said that an investigation is being carried out to discover how the worm managed to infiltrate the systems.
Klez only affects SQL servers that have not been adapted from the default settings, which is standard security practice in government departments.
The State Department emphasised that only one office has been affected by the virus.
The worm steals the email identity of its victims before passing itself on. While it does not cause any damage to files, it quickly clogs networks.
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