The few that get passed on are successful. They go on and get copied and varied again. That's how evolution works. But it's one thing to say an idea bears a resemblance to a virus, and quite another to say an idea is a kind of virus. A lot of people, scientists included, don't like the implications.
Academics quibble over memetic definitions, which remain vague.
Others argue point-blank that memetic theory can't be proved. Both groups have a point. The definitions are subject to interpretation, and thinking leaves no fossil record. Most of us, though, don't like the idea of memes because they exclude free will. In the meme scheme, a human becomes a breathing Xerox machine.