Source : Free On-Line Dictionary of Computing

sig virus
     
        A parasitic {meme} embedded in a {sig block}.  There was a
        {meme plague} or fad for these on {Usenet} in late 1991.  Most
        were equivalents of "I am a .sig virus.  Please reproduce me
        in your .sig block.".  Of course, the .sig virus's memetic
        hook is the giggle value of going along with the gag; this,
        however, was a self-limiting phenomenon as more and more
        people picked up on the idea.  There were creative variants on
        it; some people stuck "sig virus antibody" texts in their
        sigs, and there was at least one instance of a sig virus
        eater.
     
        [{Jargon File}]