Seems the infection came in via a USB device that an astronaut brought to orbit. The laptop does not have a network connection (what - no WiFi?), so the threat seems, err, remote.
Nasa has confirmed that laptops carried to the ISS in July were infected with a virus known as Gammima.AG.
The worm was first detected on Earth in August 2007 and lurks on infected machines waiting to steal login names for popular online games.
Via Slashdot, which as always offers up quality snark in the comments:
Network security really isn't that hard! It isn't like it's rocket scie... oh... nevermind...