Over the past five years the ways in which the social transmission of knowledge can fail us have come into sharp focus. Misinformation shared on social media Web sites has fueled an epidemic of false belief, with widespread misconceptions concerning topics ranging from the prevalence of voter fraud, to whether the Sandy Hook school shooting was staged, to whether vaccines are safe. The same basic mechanisms that spread fear about the tomato hornworm have now intensified—and, in some cases, led to—a profound public mistrust of basic societal institutions. One consequence is the largest measles outbreak in a generation.