There is much widespread concern about “fake news” and other forms of misinformation, and how they can undermine democracies. Recent research has begun to focus on how fake news and misinformation is spread, and the factors that determine whether information is widely shared by users, thereby going viral. Here I explore why people believe and share information that they know to be false, a phenomenon known as participatory propaganda, and how this can give rise to cascades of misinformation in which ultimately many actors are unwitting participants. I review the limited literature on participatory propaganda and sketch some possible countermeasures to this relatively new phenomenon.