Did mad cow disease actually come from humans? Alan Colchester, the doctor who raised suspicions about the Kent meat rendering plant, has one of the most disturbing theories so far.

He publishes an academic paper that suggests a grisly international trade in decomposing animal remains could have brought the disease to the UK, after human bones picked out of the Ganges in India is unknowingly mixed with the cargo.

Will there ever be an answer to the origin of BSE? Scientist John Collinge is still looking.

Written, presented and produced by Lucy Proctor