It's rare for the leader of a country to die in office. Since 2008, it's happened 13 times worldwide - but 10 of those leaders have been African. Why is it so much more common in this one continent? Large crowds carrying candles ran alongside the hearse carrying the body of Meles Zenawi, as it made its way through Addis Ababa, on Tuesday. He had died, aged 57, after a long illness. Earlier in the month, tens of thousands of Ghanaians attended the funeral of their late President, John Atta Mills, who had died suddenly at the age of 68. Four months earlier, a national holiday was declared in Malawi to allow as many people as possible to attend the funeral of the late president, Bingu wa Mutharika, who had died of a cardiac arrest, aged 78. And in January, the president of Guinea Bissau, Malam Bacai Sanha, died in a military hospital in Paris after a long illness. He was 64. So, four African leaders have died in office this year alone. Disruptive for the countries concerned, tragic for the leaders' families. But spare a thought also for the reporters. "I seem to be getting an awful lot of calls in the night telling me an African president has died," says Simon Allison, a correspondent for South Africa's Daily Maverick website. "Why do African presidents keep dying?" The question led him to take a close look at their survival rate. "Go back just a little bit further and the list of dead sitting African presidents gets alarmingly longer," he says. Indeed, since 2008, 10 African leaders have died in office.