Science is generally seen as a process bound by rigorous rules, which its practitioners must not transgress. Deliberate fraud occasionally intrudes, but it is soon detected, the perpetrators cast out and the course of discovery barely disturbed. Far more interesting are the outbreaks of self-delusion that from time to time afflict upright and competent researchers, and then spread like an epidemic or mass-hysteria through a sober and respectable scientific community. When this happens the rules by which scientists normally govern their working lives are suddenly suspended. Sometimes these episodes are provoked by personal vanity, an unwillingness to acknowledge error or even contemplate the possibility that a hard-won success is a will o' the wisp; at other times they stem from loyalty to a respected and trusted guru, or even from patriotic pride; and, worst of all, they may be a consequence of a political ideology which imposes its own interpretation on scientists' observations of the natural world. Unreason and credulity supervene, illusory phenomena are described and measured, and theories are developed to explain them - until suddenly, often for no single reason, the bubble bursts, leaving behind it a residue of acrimony, recrimination, embarrassment, and ruined reputations. Here, then, are radiations, measured with high precision yet existing only in the minds of those who observed them; the Russian water, which some thought might congeal the oceans; phantom diseases that called for heroic surgery; monkey testis implants that restored the sexual powers of ageing roues and of tired sheep; truths about genetics and about the nature of matter, perceptible only to Aryan scientists in the Third Reich or Marxist ideologues in the Soviet Union; and much more.