Scientists have traced the root of a cholera pandemic that has swept poor countries in three waves over nearly four decades - a bacterial strain that first emerged in Bangladesh.
The current pandemic is the seventh since cholera, a water- and food-borne diarrhoeal disease caused by the Vibrio cholerae bug, emerged nearly two centuries ago.
Gene sequencing of 154 samples of V. cholerae taken from patients around the world show today's pandemic can be traced to an initial outbreak of cholera in the Bay of Bengal in 1975, the investigators said.
In 1982, the strain, known as El Tor, acquired genes making it resistant to antibiotics. As a result, successive waves of the disease spread around the world, propagated from the original source.
The new probe, published in the British journal Nature, points to the likely role of modern travel in transmitting the bacteria -- and the importance of the Gulf of Bengal as a "reservoir," or source from which the germ can always be transmitted.
"Our research shows the importance of global transmission events in the spread of cholera," said Julian Parkhill of Britain's Wellcome Trust Sanger Institute.
"This goes against previous beliefs that cholera always rises from local strains, and provides useful information in understanding cholera outbreaks."
Cholera affects between three and five million people a year, with 100,000-120,000 deaths annually.
The latest prominent outbreak occurred in Haiti in October 2010 in conditions of poor sanitation after the country's earthquake.
In June this year, US investigators determined that the bacteria had been brought to Haiti by UN peacekeepers from Nepal.
The study is the latest example of the power of genomics -- the unravelling of the DNA code of an organism, whose mutations enable scientists to draw up a "family tree" to show when and where the bug changed.
El Tor probably evolved in the 1950s, said the paper.