As most viruses are indeed made for Windows and will only run on Windows as well, the chances of you catching a virus from a USB drive or some other removable drive is close to nil, mainly because viruses on external devices normally use an "Autorun" executable file to try to spread itself, and if you plug that same virus-infested USB drive onto a computer running Linux, it wouldn't do anything as Linux isn't able to run Windows .exe files (without special software like Wine), and besides, by default, any file that is downloaded to a Linux system from an external source does not have executable privileges. If you wanted to execute, say, a shell script you picked up from a friend over the Internet, you would have to give that script executable privileges through the use of "chmod +x". This is definitely much more safer than what Windows does...heck, if you rename any random text file and append it with .exe, Windows will forget it was a text file and just try to run it as an executable file.
Now, people have different tolerance levels for computer security; personally, I think UAC was one of the best steps Microsoft has ever taken in securing Windows, by making a more clearer distinction between the admin account and regular user accounts, and thus making it a more versatile multi-user platform (which Windows was never intended to be...but that's another story). Besides, Ubuntu/Mint has had "UAC" for many years with "sudo" and "gksudo". Well-defined and strict permissions and privileges, and the concept of least user access, are important concepts in Linux and many other Unix-like systems, and are essential for security. Anyways, Fedora has SELinux (Security Enhanced Linux), which extends this concept and allows you to define even more fine-grained permissions for individual programs and the like (mandatory access control instead of discretionary access control) installed by default, but you can do the same with Ubuntu and Mint. Ubuntu comes with AppArmor by default; Mint comes with neither. You can pick up either SELinux/AppArmor from Synaptic/apt-get if you're interested, although learning how they work can be a bit troublesome in the beginning, especially if you're new to Linux.
Debian Testing x64/LM9 Main x64/Windows 7 x64 - LG R580 laptop w/ Intel Core 2 Duo T6500 2.1 GHz, 4 GB DDR2 RAM, Nvidia Geforce G 105M, Ralink rt2860 802.11n, 300 GB WD HD 5400 rpm