Memetic hazards are extreme bad memes - or collections of bad memes. They are sources of culturally-induced harm.
I don't have much to add to Aetheric Research's description of a "memetic hazard" - so I'll just quote it:
A memetic hazard is defined as information with three main attributes. The first attribute is that it spreads from person to person, whether through personal contact or some form of recording. The second attribute is that this information causes some form of distress, whether as benign as mental stress to the individual or as dangerous as societal dysfunction. The third attribute is that it must cause preoccupation–that is to say, it maintains sufficient presence in the host’s mind that either a significant portion of his attention remains focused on it, or it plays a significant part in his decision-making process.I might be inclined to play down the significance of the last attribute. One important way of measuring memetic hazards is by their severity. Some memetic hazards waste a small quantity of time. Others take over people's lives and ruin them.
Propaganda represents a memetic hazard on a national scale.
One academic source that covers memetic hazards is Nick Bostrom's Information Hazards: A Typology of. Potential Harms from Knowledge. Nick focuses on harm from accurate knowledge. Of course, most memetic hazards are not of this kind.
Memetic hazards are a type of information hazard. Want to be specific? Use the term 'memetic hazard'. Want a more general term that also covers individual learning? Use the term 'information hazard'.
Memetic hazards are not necessarily harmful to all. Some will have strong memetic immune systems. Some may actively benefit in some way from propagating them.