The word "meme" was created by Dawkins to be similar to "gene", because they spread similarly. Cancer is, in a certain sense, a genetic disease, the result of the malfunction of genes caused by mutation. So, to continue the analogy, a badly mutated meme can be like cancer, and can spread like one.
Certain unhealthy ideas throughout history seem to metastasize and cause great harm to their host societies until they are eventually discarded or defeated.
What do you have in mind? I can think of few modern examples that resulted in failed states and losers in wars, but many other bad ideas persist (but identifying them as such can be socially and politically sensitive) without dying off.
Memes aren't cancer, but I agree that they can be pathogenic.
