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.