I am also curious.
this is a first if correct.
though I highly doubt the n900 has an actual virus.
Who in their right mind would take the time to create a "big virus" for the n900?
It's like making a virus for the Commodore 64.

Even if done out of a desire to be malicious......it would hardly impact enough people for the twisted enjoyment.
That is just along the lines of the mindset behind a virus creation...

If it were a "big virus" that was apparently openly attacking and disabling n900's ...why haven't other reports been noted yet?
unless this is the very first report...but I have doubts there too.

most likely it is the host machine os.
How knowledgeable is the n900 user with the maemo platform and devices?
I wonder, since it is has been common knowledge for many many years that the download links here for the firmware has been dead.
Has the owner of this n900, bought or inherited the device from someone else...and inherited the past owners problems....
or at best ...
hasn't done a firmware reflash since (since it is obvious) the links for the firmware here went down ...so many years ago ...and the n900 in question is simply in need of a fresh flash?

edit:
Most likely... as has been mentioned... Windows host machine perhaps found a file upon establishing a connection and scan of the mass storage of a virus in the mass storage ... a file or program that would impact Windows....but that has no bearing or affect ...and means nothing to maemo linux.
Just remove the offending file....or reflash