Most African herbalists cause no more damage than dispensers of alternative medicines on our high streets. Every now and then, however, a sinister practitioner will advise a very special client that while roots and animal parts are useful, the most potent medicines are made from human blood, liver, spleen and heart. Yes, it is dreadful, he whispers, but there are unscrupulous people about, and I have heard that your rival is in the market for the stuff. What choice do you have? When one big man is persuaded, his peers are immediately alerted. In consequence medicine murders tend to crop up in clusters, the clients typically rich and powerful men. The anti-human sacrifice and trafficking unit of the Uganda police recorded 26 cases in 2008 and 28 in 2009, and a number of suspects were brought to trial. Enter Tim Whewell of the BBC’s Crossing Continents programme.