A biological virus is a strand of DNA encased in a protein coat that injects the DNA into living cells where it convinces the cell's genetic machinery to produce copies of the virus and release them so they can infect other cells. A meme virus, by analogy, is a meme (a thought, a piece of text, a joke, a tune...) that convinces the person it "infects" to spread the meme to others. The simplest meme virus is probably the sentence "Copy me!", but it's not very effective because there's no incentive to copy it. A more viable meme virus is "Copy me or you will have bad luck!", which is exactly how every chain letter works. Some have called the phrase "meme virus" a pleonasm because all memes are self-replicating, but I think there is a difference between a normal meme and a viral meme analogous to the difference between cellular DNA and viral DNA, namely that the only function of both viral DNA and viral memes is self-replication.