The Jargon File (version 4.4.7, 29 Dec 2003):

sig virus
 n.

    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.


The Free On-line Dictionary of Computing (30 December 2018):

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]