Using bits of human intestine stored in a Philadelphia medical museum in 1849, scientists have decoded the genes of an early form of cholera, the deadly diarrheal disease that first swept the globe just a few decades earlier.

The disease is still a lethal menace, as was shown in Haiti four years ago, when an unexpected outbreak after an earthquake killed more than 8,000 and hospitalized hundreds of thousands more. But it has evolved since the 19th-century pandemics, which killed millions; the new work, by scientists at McMaster University in Ontario, creates the first chance to study the genome of the pandemic “classical” strain and understand its roots.

Cholera experts at Harvard and Johns Hopkins praised the work, although they disagreed about what it proved about the disease’s recent history....