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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.