In the wake of last year’s earthquake in Haiti, cholera arrived on the island for the first time in 60 years. According to the World Health Organization, 419, 511 Haitians got sick with cholera as of July 31, of which 5,968 died. The infection rate is dropping right now, but the arrival of Hurricane Irene could change that.

As I wrote in December, scientists applied evolutionary biology to find clues to how cholera–or, more precisely, the bacteria Vibrio cholerae— came to Haiti. They compared the DNA in the strain in Haiti to ones that have been found in other parts of the world. From this analysis, they drew a tree, which I’ve reprinted below.

The bacteria in Haiti was