A major cholera pandemic has spread in at least three waves from a single global source: the Bay of Bengal.A study in Nature reveals cholera's spread over the last 60 years into Asia, Africa, Europe and the Americas, continent-hopping on long-haul flights.The research by a team from Cambridge's Sanger Institute showed the infection is evolving, with the newest waves showing antibiotic resistance.A UK expert said it was "a scandal" cholera was still affecting people.Cholera is a bacterial infection of the intestine that causes diarrhoea. It affects 3-5m people annually in 56 countries, killing between100,000 and 150,000.If untreated, it can kill within hours through dehydration. It is easily treated by drinking clean water, but without this, severe cases have a 30-50% mortality rate.'Only explanation'In this study, the researchers sequenced the genome of 154 samples collected from patients around the world. Genome sequencing technologies have been getting better, faster and cheaper. Until recently, sequencing would be carried out on just four or five bacteria samples.Similarities between cholera genomes showed how the various strains are related, while subtle differences showed how it is evolving.By investigating these bacteria at the genetic level, the authors were able to piece together the story of the latest, and ongoing, global cholera pandemic."We were surprised to see that the pattern we see is very clear. All of the samples were related. There is a single global source of cholera in the Bay of Bengal," said co-author Dr Nick Thomson of the Sanger Institute.It is not yet clear why the Bay of Bengal is at the centre of the pandemic, though cholera bacteria exist naturally within some marine ecosystems.The local ecology, climate, and the presence of large river deltas are likely to be key factors in its presence there.The results show several cases of cholera suddenly jumping between continents, suggesting that it was spread by passengers on long-haul flights."I think that's the only possible explanation. Our data show that this has happened, for example from Angola to South America."Many people can have cholera with no symptoms, so they transmit it without realising," added Dr Thomson.'One fell swoop'The most recent outbreak is in Haiti where it took hold shortly after the 2010 earthquake after being absent for more than a century.So far it has killed nearly 6,000 people and hospitalised 200,000.