President Hugo Chavez of Venezuela thinks the US may have developed a secret technology to give him and other Latin American leaders cancer. He said the fact that several presidents have had cancer is "difficult to explain using the law of probabilities". Is he right? Tim Harford speaks to Dr Eduardo Cazap, president of the Union of International Cancer Control. Plus, it is often said that there are more Malawian doctors in the British city of Manchester than there are in Malawi. Can this be true? And if professionals emigrate is it always bad news for the country they leave? The programme hears from John Lwanda, a Malawian doctor based in the UK; and Robert Guest, author of "Borderless Economics". This programme was originally broadcast on the BBC World Service.