There is a security program in the repo called clamav. You should install clamav, clamav-db, clamd, clamtk, and libclamav6. (Some of them may be marked for install
automatically when you select clamav.) Then, in a terminal, you run (from root) freshclam. This is to bring the database up to date.The first time you do, it will tell you
to modify a text file, so you do that by putting a # in front of the word "example" about 3 or 4 lines from the top, and then saving the file. You will find that the GUI
complains about not being up to date, but that doesn't matter--the program will work fine if you always run freshclam before you start it. In a period of a couple of years,
I've only seen it complain once or twice, and it was not a "real" virus. (It also has some test files that it will catch, depending on what you scan.) But anyway, for peace
of mind, I scan the system once in a while when I think of it. Can't hurt!