USB Pen Drive Infected By W32.Virut variant

Just been to help someone with an infected PC - Virut and Thermida were the main problems. I carry a USB Pen Dive with various bits and pieces to help remove these things. I only copied stuff off the Pen Drive and onto the PC but now a lot of those files on the Pen Drive are infected with the Virut virus!

Fortunately NOD32 picked it up and I've now formatted the drive.

How was it able to do that?

 

Maybe he had an autorun trojan infected with Virut so it was copied to the usb stick as soon as you plugged it in.

 

Funny thing is it ONLY infected the files i was using! Other exe's i didn't copy to the PC didn't get infected....

Having thought about it each program is in it's own folder and so i suppose as i open each folder the contents could be infected. Hence only the used files are infected.