"Language is a virus," William S. Burroughs once famously said, and he got most of it right.

Any idea is a virus. If it can be communicated between thinking minds, or if it is a design in nature like the DNA of a "real" virus, it can find a host and spread itself. In fact, the host spreads it, thinking it to be advantageous to do whatever is done that causes it to spread, whether that is a network connection or a sexual act.

A virus is distinct from a meme in that a virus must take control of something in order to be passed on.