One of a six-part series of documentaries offering new insights and debate on topical science issues. The only way to prove that variant CJD is caused by BSE is to inject infected material from a cow into a human and sit back and wait to see whether, in ten, twenty or even forty years time, that person develops vCJD. Without this crucial experiment, there is still no direct proof that variant CJD is caused by BSE. This programme examines what causes BSE. Metals, organophosphates, tiny viruses called virions have all at one time been implicated. The current front runner is rogue prions, but are these elaborately folded proteins found in all species of animals the real cause?